int. j. lang. comm. dis., 2000, vol. 35, no. 2, 165–188
Prevalence and natural history of primary
speech and language delay: Ž ndings from
a systematic review of the literature
James Law†*, James Boyle‡, Frances Harris†, Avril Harkness‡ and
Chad Nye§
† Department
of Language and Communication Science, City University,
London, UK
‡ Department of Psychology, University of Strathclyde
§ Department of Communication Disorders, University of Central Florida
(Received July 1998; accepted March 1999)
Abstract
The prevalence and the natural history of primary speech and language delays
were two of four domains covered in a systematic review of the literature
related to screening for speech and language delay carried out for the NHS in
the UK. The structure and process of the full literature review is introduced
and criteria for inclusion in the two domains are speciŽ ed. The resulting data
set gave 16 prevalence estimates generated from 21 publications and 12 natural
history studies generated from 18 publications. Results are summarized for six
subdivisions of primary speech and language delays: (1) speech and/or language,
(2) language only, (3) speech only, (4) expression with comprehension,
(5) expression only and (6) comprehension only. Combination of the data
suggests that both concurrent and predictive case deŽ nition can be problematic.
Prediction improves if language is taken independently of speech and if expressive and receptive language are taken together. The results are discussed in terms
of the need to develop a model of prevalence based on risk of subsequent
diYculties.
Keywords: primary speech and language delay, children, prevalence, natural
history, epidemiology.
Introduction
The present review was commissioned and managed by the National Health Service
Centre for Reviews and Dissemination at the University of York on behalf of the
*Address correspondence to: James Law, Department of Language and Communication Science, City
University, Northampton Square, London EC1V OHB, UK; e-mail: J.C.Law@city.ac.uk
International Journal of Language & Communication Disorders
ISSN 1368-2822 print/ISSN 1460-6984 online © 2000 Royal College of Speech & Language Therapists
http://www.tandf.co.uk/journals/tf/13682822.html
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J. Law et al.
National Health Technology Assessment Programme of the NHS in the UK. The
overall aim of the review was to establish whether there was a case for the
introduction of universal population screening for speech and language delay. An
epidemiological model was adopted (Law et al. 1998) and this led to the identiŽ cation
and review of the four key domains of the literature: prevalence, natural history,
intervention and screening. Although these domains have been treated independently in the literature to date, the position taken in the review was that they are
closely associated. While it may be possible to calculate prevalence at any age given
a suitable measurement, it is only a meaningful construct in the context of natural
history of the condition. Similarly, both screening and intervention are tied into
the need for a clear concept of case deŽ nition and appropriate targeting depends
on an understanding of the course of the disability. The intervention and screening
literature are reviewed elsewhere (Boyle et al. in preparation, Law et al. in press).
Prevalence
Prevalence is conventionally expressed as the proportion or percentage of cases in
a given population at a speciŽ ed time. Normal rather than clinical population is
the focus in the present review. Prevalence diVers from, but is often confused with,
incidence, which refers to the number of new cases (Mosciki 1984). Cases may move
out of the pool from which prevalence is taken because of mediating factors (such
as spontaneous recovery, intervention or death), but prevalence also depends upon
incidence. Prevalence will rise if incidence exceeds the decrease brought about by
these mediating factors, and is an important concept for three reasons: it allows
planning of service delivery; it should also allow calculation at an epidemiological
level of the impact of intervention, with a successful intervention being one that
results in a decline in prevalence; and it should re ect current knowledge about a
disorder where the boundaries lie between for example normality and abnormality
and which cases respond to intervention. To date there has been no attempt to
carry out a systematic synthesis of the prevalence literature although there have
been some extensive narrative reviews (MacKeith and Rutter 1972, Healey et al.
1981, Silva et al. 1987, Harasty and Reed 1994).
Natural history
‘Natural history’ is a speciŽ c term used to describe the prognosis of a condition in
the absence of intervention (Gordis 1996). It can be a diYcult concept when it
comes to children’s development because, in most cases, children receive some
form of intervention by virtue of entering the education system. Nonetheless there
probably is a distinction to be drawn between intervention of a generic nature and
intervention which is targeted at the child’s speciŽ c diYculties. Natural history is
important because the anticipated status of a putative case allows determination of
current status. Thus, if a two cases are identiŽ ed as having comparable levels of
skill at time one but it is known that there is a much greater risk of persistent
problems for case one than for case two, then the relative weighting attributed to
case one will be higher.
Prevalence and natural history of primary speech and language delay
167
Systematic review of the literature
The aims of a systematic review are to locate, appraise and synthesize evidence
from studies to provide empirical answers to scientiŽ c research questions. It diVers
from other types of review in that it adheres to a strict scientiŽ c design to make it
more comprehensive, and to minimize the chance of bias (NHS Centre for Reviews
and Dissemination 1996). Such a review follows a strict procedure that involves
the identiŽ cation of published and unpublished data sources, the development of
research questions, search strategies and inclusion criteria, the reliable coding of
the data, and its synthesis ultimately leading to data-driven conclusions.
Methodology
Scope of the literature
The literature for 1967 to May 1997 was targeted. CROS AND DIALINDEX
search mechanisms identiŽ ed the databases most likely to identify appropriate
literature. Thus, six databases were identiŽ ed: Embase, Medline, ERIC, PsychINFO,
CINAHL and LLBA. In addition, the System for Indexing Grey Literature in
Europe (SIGLE) and the Boston Spa Conferences (British Library Databases) were
searched for unpublished, or ‘grey’, literature. Grey literature is scanned to attempt
to overcome publication bias. Finally, journals were hand-searched to identify
relevant authors in the Ž eld. In all, 53 prevalence papers met the criteria of relevance
for prevalence but of these only 21 met the full inclusion criteria. These provided
16 sets of data. Twenty-three papers met the initial criteria for relevance in the
natural history domain, but of these only 18 were included in the review providing
12 data sets (see Appendices 1 and 2).
Criteria for inclusion in the review
The review dealt with children with primary speech and/or language delays. That
is, it did not include children with delays secondary to other conditions such as
autism, or more general developmental disabilities. It is recognized that, in the
absence of universally recognized criteria for primary delays, this may be a diYcult
judgement to make and that there may be more similarities than diVerences between
these groups. Nonetheless the distinction between primary and secondary delay is
one that is widely recognized by both researchers and clinicians and, therefore, has
face validity. Authors use many diVerent ways to deŽ ne the populations they
describe. Some use discrepancies between measures of language and general abilities,
but again there is no widely accepted level of discrepancy that could be imposed
on the data. In addition, some design criteria were speciŽ ed for each domain.
Prevalence studies
Studies were included that estimated the prevalence of speech and language delays
in children aged up to 16 years in a general population. The studies needed to
present data about the number of participants and the diagnostic samples, and the
deŽ nition of case status had to be determined either by standardized measures of
speech and/or language or to use clearly deŽ ned clinical judgement. Studies of
clinic populations were excluded. In addition, a distinction has been drawn between
168
J. Law et al.
single-level prevalence studies, which have been based on surveys or clinical judgement, not subjected to external validation, and those studies that have been based
on a pre-screen or net of the population with proportions of passes and fails
sampled and then given a diagnostic assessment, either on a standardized language
procedure or on a criterion referenced clinical judgement (which the authors have
made some attempt to validate or deŽ ne such that it could be replicable). The
‘single level designs’ have been excluded on the grounds that they would be
impossible to replicate and are likely to lead to under-reporting in all but the most
clear-cut medical cases (Leske 1981). Many of the largest studies have been excluded
because, although the sampling was potentially of considerable interest, their singlestage approach would not have met the inclusion criteria for the review. They have
also tended to report the lowest prevalence Ž gures (Blum-Harasty and Rosenthal
1992). This two-stage criterion resulted in the exclusion of a number of studies that
are commonly included in narrative reviews most notably Drillien and Drummond
(1983), Fundudis et al. (1979), Hull et al. (1971), Morley (1965) and Peckham (1973).
The reason for applying such a stringent criterion is that it provides a measure of
control against threats to the reliability of the results and hence to the validity of
the interpretation.
Natural history
Large-scale prospective same-age cohort longitudinal studies oVer the best evidence
of natural history outcomes. However, most of the prospective studies located by
the review were those where at least a proportion of the subjects received speech
and language therapy services. A statement regarding the numbers in therapy, or
the amount of therapy received, led to the study being excluded from this review.
‘Therapy’ was interpreted in the context of the studies as speciŽ c advice or
assessment from speech and language therapists. Where there was no such statement
about therapy contact for the subjects, the study was included. In a minority of
instances a study explicitly stated there was no intervention for the subjects. General
advice given routinely by a health professional other than a speech and language
specialist was regarded as non-speciŽ c intervention, and such studies were included.
This applied, for example, to some treatment studies where control groups received
only general help. Moreover, within a prospective design with some subjects receiving treatment, where data for individual untreated subjects could be retrieved, their
outcome was noted. Another criteria for inclusion ensured that there was a retest
interval of at least 6 months, using norm- or criterion-referenced language outcome
measures. No minimum number of subjects was set.
These constraints, in particular the stringent inclusion criteria, also resulted in
a number of well-known studies being excluded from the review (e.g. Sheridan and
Peckham 1975, Klackenburg 1980, Paul 1993). While their designs have many
exemplary features, and have achieved long-term follow up for relatively large
samples, it has not proved possible to separate out possible treatment eVects in
their data set.
Reliability
To ensure that the same data was being extracted appropriately, two independent
coders coded a subset of the papers and the percentage agreement rate was reported.
Nineteen per cent of the prevalence papers and 14% of the natural history papers
Prevalence and natural history of primary speech and language delay
169
were subjected to the reliability check resulting in reliabilities of 84.8 and 85.4%
respectively. DiVerences were resolved following discussion between the coders.
The majority of sources of diVerence between the coders arose as the result of
gauging which Ž gures to use as the denominator at any one stage in the calculation
of prevalence or natural history because of diVerential attrition in the populations
concerned.
Results
Results from prevalence studies
To represent its diversity the data in the present review were classiŽ ed by the
language domains measured, and by the age of the children in the sample. Table 1
shows prevalence for (1) speech and/or language, (2) language and (3) speech.
Table 2 focuses on those studies that have looked at language delay in the absence
Table 1.
Median prevalence estimates by type of speech and language delay and age
Type of delay
Speech/language delay, language delay only and speech delay only
Age (years)
2;0
3;0
4;6
5;0
6;0
7;0
Speech/language delay
median of estimates
(range)
Language delay only
median of estimates
(range)
Speech delay only
median of estimates
(range)
5.0 a
6.9 [5.6–8] a,c,e
5.0 [–] a
11.78 [4.56–19.0]b†
–
–
16 [8–19] f
2.63 [2.27–7.6]g,h,k
–
6.8 [2.14–10.4] b,g,i,j,d
5.5 d
3.1 [2.02–8.4] g,d
–
–
–
7.8 [6.4–24.6]b,d,j
14.55 [12.6–16.5]d,j
2.3 d
a Bax
et al. (1980, 1983); b Beitchman et al. (1986); c Burden et al. (1996); d Dudley and Delage (1980);
et al. (1974); f Rescorla et al. (1993); g Silva et al. (1983); h Stevenson and Richman (1976),
Richman et al. (1982); i Tomblin et al. (1997); j Tuomi and IvanoV (1977); k Wong (1992).
† Beitchman et al. (1986) is the only study to include prevalence estimates for both speech and language
and speech or language. This highlights the diYculty in synthesizing data in this area because both
the level of diYculty and the extent to which the categories can reasonably be teased apart from the
data is unclear.
e Randall
Table 2. Median prevalence estimates by type of language delay and age
Type of delay
Expressive and receptive language delay, expressive delay only and receptive delay only
Age (years)
2
3
5
7
a Silva
Expressive and
receptive language
delay
median percent (range)
Expressive delay only
median percent (range)
Receptive delay only
median percent (range)
–
3.01a,b [2.63–3.4]
2.14a
2.02a
16 c [8–19]
2.3 a,d [2.27–2.34]
4.27a
2.81a
–
2.63a
3.95a
3.59a
et al. (1983); b Wong (1992); c Rescorla et al. (1993); d Stevenson and Richman (1976).
170
J. Law et al.
of speech delay and separates those language delays out into expressive/receptive
delays, isolated expressive delays and isolated receptive delays. This group represents
a subgroup of those quoted in table 1. Table 3 displays studies that report prevalence
across diVerent age groups and which are impossible to combine in a meaningful
way. In a number of studies more than one data set is provided for a given age
band. To avoid over-representing such studies in the total data set only a single
(median) Ž gure was included for each study.
Speech and/or language delay
The largest single group and, thus, the group most likely to be picked up in the
process of early identiŽ cation (the original aim of the systematic review) is that
with combined speech/language diYculties, where researchers have included the
possibility of either speech or language being implicated. This corresponds to
column 1 of table 1, and the median for this column is 5.95%. However, although
the median was adopted as the most appropriate measure of central tendency in
view of the variability observed in the data, considerable caution needs to be taken
in extrapolating to produce single composite prevalence estimates especially given
the relatively small number of studies involved.
Taking speech and language delay as a single construct, the majority of studies
took diagnostic assessment score cut-oVs between Õ 2 and Õ 1.5 standard deviations
(SD) below the mean for the standardization sample on a single measure, which
automatically gives prevalence rates of between 2.28 and 6.68%. In most cases the
range of Ž ndings is slightly wider than would be anticipated on this basis, ranging
from 1.35 to 8% (table 1, column 1; table 3). Where more liberal cut points are
adopted higher prevalence Ž gures result and this is compounded when failure on
any one of a number of measures is used as the criteria. For example, one of the
highest Ž gures came from a large population study that used multiple measures
( Beitchman et al. 1986). The Ž gure for speech or language was much higher than
most other estimates (19%) being an inclusive estimate for speech or language delay
and using a cut-oV of Õ 1 SD below the mean on a series of diVerent speech and
language measures. By contrast, a scale of clinical diYculty adopted by Paul et al.
(1992) (table 3) reported much lower Ž gures (1.35% for mild, moderate and severe
cases, 0.65 for moderate and severe or what they term ‘serious’ cases only). These
Ž gures derive from a study in which case status depended upon a single objectiŽ ed
criteria for clinical judgement rather than performance on standardized assessments
of speech and language performance. Such conservative criteria may re ect the fact
that this study was carried out in Jamaica, a country where services, with fewer
Table 3.
Period prevalence estimates
Age
(years)
Study
Type of delay
Prevalence estimate
(%)
2:0–9.0
6.0–12
3:0–5.0
5.0–7.0
6.0–12
12–14
Paul et al. (1992), Thorburn et al. (1991)
Harasty and Reid (1994)
Stewart et al. (1986)
Kirkpatrick and Ward (1984)
Harasty and Reid (1994)
Warr Leeper et al. (1979)
Speech and Language
Speech and Language
Speech
Speech
Speech
Speech
1.35
8.0
1.5
4.6
12.6
7.3
Prevalence and natural history of primary speech and language delay
171
resources available, would be likely to apply a more stringent criterion as to who
should receive services.
Language delay only
Language delay in the absence of speech delay is the group most commonly
identiŽ ed in the studies included in the present review. Some studies have reported
children with either expressive or receptive delays as one group (table 1, column 2;
table 2, column 1). The variability here is also wide, giving a range of 2.02–19%.
In one case a cut-oV, which would anticipate a higher prevalence estimate
( Õ 1.25 SD), resulted in a convergent prevalence estimate of 7.4% because children
had to receive two low test scores to achieve case status (Tomblin et al. 1997). The
Ž gures for expressive with receptive language delays combined show little variation
within a narrow band, re ecting the conservative cut-oV scores adopted in these
studies and perhaps a greater conŽ dence in the most appropriate level at which to
set case status. It is of note that this most recent population study suggests a higher
level of prevalence for children with language problems based on a discrepancy
criterion rather than for the broader category of children with primary delays
captured by most of the other studies. It is probable that the rather more liberal
cut point used in this study explains the variation. But it would be wrong to over
emphasize the diVerence. The important issue is whether the more inclusive deŽ nition of language delay results in more cases whose diYculties would not otherwise
resolve being provided with appropriate intervention.
Most notable is the sharp drop between 2 and 3 years, the high early Ž gure created
by the measure of output vocabulary adopted by Rescorla et al. (1993). In the main
the estimates use either single measures of language which amalgamate diVerent
components (comprehension, expression, vocabulary, verbal memory, etc.). Sometimes
cases are identiŽ ed by failure on any one of a number of subtests (Stevenson and
Richman 1976, Dudley and Delage 1980). In other cases composite measures made
up of a number of diVerent tests are used (Tomblin et al. 1997). The evidence seems
to suggest that it does not make much diVerence which approach is adopted. It is
noteworthy that the level of prevalence rises at the 5-year mark. In part this is the
function of the 10.4% Ž gure reported by Silva et al. (1983), which sums expressive,
receptive and expressive receptive cases. These are separated out in table 2, column
1. It is also possible that there is a change in expectations at this point in the child’s
development, a closer approximation to the adult norm is now expected and as the
expectations increase so do the number of potential cases of delay.
In a study of primary speech and language delay it is obviously important to
establish what is known about the prevalence of speciŽ c language impairment (SLI),
that is language impairments that are identiŽ ed by means of the discrepancy between
language and IQ measures. Only two studies have achieved this with very disparate
results. Stevenson and Richman (1976), who obtained a prevalence of 0.6% for
SLI, and Tomblin et al. (1997), who obtained 7.4% for a group of SLI children.
This diVerence is likely to stem from the diVerence in discrepancy criterion adopted
and the age of the children in the study. Thus, Tomblin et al. speciŽ ed that IQ had
to be within the normal range with language level below Õ 1.25 SD on their
composite measure at 5 years, while Stevenson and Richman speciŽ ed that expressive
language had to be less than two-thirds of mental age, which in turn had to be
two-thirds of chronological age at 3 years of age.
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J. Law et al.
Speech delay only
The Ž gures for speech (not language) delay are again highly variable, ranging from
2.3 to 24.6% (table 1, column 3; table 3). Kirkpatrick and Ward (1984) identiŽ ed
4.6% using a Õ 2 SD cut-oV. Beitchman et al. (1986) set a Õ 2 SD cut-oV on
particular subtests to reach their speech prevalence rate of 6.4%. In contrast, Tuomi
and IvanoV (1977) adopted a criteria of 1 year behind chronological age and
reported 24.6% in kindergarten children, although this fell to 16.5% by Grade 1.
These Ž gures are relatively high and probably re ect the relatively liberal deŽ nition
of what constitutes a case, including include children with poor ‘stimulability’,
multiple and consistent errors and those with a lateral lisp. It would appear that
the childrens’ speech is reported in absolute rather than developmental terms.
SigniŽ cantly prevalence studies of speech delay in the pre-school years did not fulŽ l
the inclusion criteria for the review. This probably re ects a recognition of the
diYculty in establishing case status during the very early years.
Language only—expressive and receptive delay
Two studies have reported data that allow combination of expressive and receptive
language scales (Silva et al. 1983, Wong et al. 1992). It might be anticipated that
these data should coincide with those from table 1, column 2. In fact, this method
of classifying the children suggests a rather more conservative estimate of prevalence
as between 2 and 3%, which remains constant across the age range. To classify
children in this way it is necessary to use standardized procedures that tease apart
expressive and receptive skills. But inevitably the precision using measures in this
way eVectively exacerbates the circularity associated with the use of this type of
metric, a point discussed further below.
Language only—expressive delay
Three authors report Ž gures for delays only in expressive language skills. Two
studies use only an expressive language measure (Stevenson and Richman 1976,
Rescorla et al. 1993), while Silva et al.’s criteria (1983) seek to identify expressive
delay in the absence of delay in receptive language skills. The Ž gures reported by
Stevenson and Richman, and Silva range from 2.34 to 4.27% over the age range
3–7 years. The highest Ž gure comes from Rescorla et al. (1993), with 8, 16 and
19% according to the screen cut-oV. These Ž gures were computed from a singlevocabulary checklist rather than a diagnostic test performance, which was the
approach adopted in the other studies. That is, the range of reported prevalence
re ects the initial screening stage of this study. Alternative prevalence Ž gures can
be computed re ecting the reference measure, giving 9.8 and 13%. However, the
age range for Rescorla et al.’s study was at least 1 year below that of the other
studies, and it may be that the range of expressive vocabulary development is
especially wide in the slightly lower age range.
Language only—receptive delay
The Ž gures for receptive delay are again tightly grouped ranging from 2.63 to
3.95%. All the Ž gures come from the Dunedin study (Silva et al. 1983). Silva et al.’s
Ž gures here could include children who are actually expressive-receptive language
Prevalence and natural history of primary speech and language delay
173
delayed, but who just achieved a pass on the expressive language measure while
failing the receptive test. The Ž gures may, therefore, be an instance of psychometric
convention for cut-oVs confounding the clinical impression.
Relying on standardized assessments to establish case status makes it diYcult
to judge whether prevalence decreases with age. A given cut-oV on such a measure
will result in the same percentage of the population being identiŽ ed at any given
time. The data presented here suggest that prevalence does not decrease over time.
Bax et al. (1983) indicate fairly stable Ž gures over the 2–4.5-year age range for their
‘deŽ nitely abnormal’ group. It seems likely that the majority of studies reviewed
above take this latter relatively stable group as their subject pool. This could suggest
that similar prevalence Ž gures across a time range re ect the same group of children
at diVerent points in time. In fact, the evidence from Silva et al. (1983) is that
individual children may move in and out of the disordered group. This may be true
or it may be a function of the lack of stability of the metrics adopted. Predictability
of problems depends upon the presenting symptoms at any one point, children
with expressive and receptive delays being much more likely to have persistent
problems than children with expressive delays alone. By contrast, Bax et al.’s ‘possibly
abnormal’ group shows prevalence decreasing from 17 to 12 to 7% between 2 and
4;6 years. This study suggests that there may be a group of children for whom
development is especially variable in rate but that may have less entrenched problems. These problems may tend to reduce over time perhaps for no other reason
than test/retest error or regression to the mean (Robson 1993). But a further
possibility is that this reduction comes as a result of the eVect of speech and
language therapy services, a point made by Butler (1989) with regard to the Bax
et al. study.
Confounding factors
There are potentially confounding factors (most notably, gender, socio-economic
status and bilingualism) in the literature that need to be considered. A propensity
for marked speech and language delays to be more common in males than females
is generally conŽ rmed by the studies reviewed here. Gender ratios derived are 1.25:1
( Randall et al. 1974), 2.26:1 (Stevenson and Richman 1976), 2.30:1 (Burden et al.
1996), 1.25:1 for both speech and language at 4 years (Stewart et al. 1986) and 2.3:1
(speech), with 1.2–1.6:1 (language) (Tuomi and IvanoV 1977). There are two
exceptions to this pattern. One is Beitchman et al. (1986), who found the reverse
pattern for speech only (0.98:1), language only (0.98:1) and speech or language
(0.82:1), and a most unexpected 0.46:1 for the speech and language diagnosis. The
other is Tomblin et al. (1997), who suggest that while boys are more likely to present
with SLI, the ratio is nearer equivalence. There are possible explanations for these
Ž gures. The Ž rst is suggested by the design of the Beitchman et al. study that sought
to sample and then project the false-negatives back into the original population
sample. Of the false-negatives, the majority was girls and in projecting back up to
the main sample the authors projected the gender balance as well as the number
of cases. A second explanation is that the relatively liberal cut-oV eVectively misses
the commonly observed discrepancy between the genders because those cases found
may be less likely to be true clinical cases and as such may tend to re ect the
normal gender balance in the population. A third explanation and one favoured by
Tomblin et al. is that existing data are the result of underreporting of diYculties in
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J. Law et al.
girls, a phenomenon which has also been reported in the literature related to reading
disabilities (Shaywitz et al. 1990). The fact that the other major cohort study in this
area did detect the predicted imbalance (2:1) adopting the Ž fth percentile as a cutoV suggests that it may be the cut-oV which is the determining factor here rather
than a high level of undetected diYculties in girls (Silva 1980).
The studies quoted here are not generally helpful in addressing the issue of
increased prevalence in lower socio-economic groups. Researchers have commented
on this issue but the inclusion criteria for this review eVectively excluded groups
that might be considered as lower socio-economic status. Some of the studies were
carried out in areas with a relatively advantaged population (Burden et al. 1996,
Rescorla et al. 1993). Bax et al. (1983) commented on the cumulative eVect of low
SES on language delay. Similarly Harasty and Reed (1994) give some indication of
the potential eVect of introducing variation of this type into prevalence estimates.
For example, they found higher rates of refusal when making up their diagnostic
sample of lower SES children.
Also the data here do not address bilingual or ethnically diverse populations.
Stevenson and Richman (1976) deliberately excluded non-indigenous families from
their study. Tomblin et al. (1997) noted increased prevalence of language delays in
monolingual African-Americans, a Ž nding not replicated by Stewart et al. (1986) in
the only reviewed study explicitly to target prevalence in a black population. Wong
(1992) found levels of language delay for children from Hong Kong comparable to
those in other studies.
In the review no studies were found, with the exception of Tomblin et al. (1997)
(also Records and Tomblin 1994), which attempted to integrate clinical judgement
and standardized procedure and to determine the eVect that this might have on the
estimate of prevalence. Accordingly, it is necessary to interpret the above results as
being based on a notional psychometric convention. One would argue that this
may tend, especially when seen in terms of a cut point of more than 1 SD below
the mean, to lead to a relatively liberal cut-oV which may, in turn, overestimate real
need. However, it does not help one to escape the essentially circular nature of
statistically derived prevalence estimates.
Results from natural history studies
A range of diVerent Ž ndings is reported, namely those relating to speech and
language, language only and speech-only delays. As above, the criteria for determining what is a case is established by the authors of the papers in question, and the
disparate nature of some of the Ž ndings probably re ects these criteria as much as
they do real diVerences in the groups. This section summarizes each category in
turn reporting median Ž gures for each category. Persistence here is deŽ ned as the
number of children who remain cases of speech and/or language delay at the
longest time point quoted by the study. Importantly only speech and language
outcomes were used rather than broader educational outcomes or behaviour
problems on the grounds that these might result in a more comparable data set.
The median persistence Ž gures for studies examining speech only, language
only, and speech and language only are given in table 4. The individual studies are
listed with their persistence estimates in table 5. Persistence here refers to the
proportion of children whom the authors reported continued to meet their criteria
for case status. The median Ž gures for studies examining expressive/receptive,
Prevalence and natural history of primary speech and language delay
175
Table 4. Median persistence reported for speech and language, language only and speech
only
Type of delay
(no. of studies)
Speech only (3)
Language only (10)
Speech and language (1)
Median persistence
(%)
Range of reported
persistence (%)
Age range
(years;months)
50
66
38
22–54
0–100
n/a
4;10–33;0
0;10–7;0
3;0–7;0
Table 5. Persistence for individual studies: speech only, speech and language, and language
only
Cases/sample size
Speech and language
Fiedler et al. (1971)†
Median
Language
(expressive and receptive)
Hall et al. (1993), Hall (1996)
Rescorla and Schwartz (1990)
Richman et al. (1982)
Scarborough and Dobrich (1990)
Silva et al. (1983)†
Thal and Tobias (1992), Thal and Bates (1998)
Klee et al. (1998)
Ward (1992)
Ward (1992)
Ward (1992)
Median
Speech
Bralley and Stoudt (1977)†
FelsenŽ eld et al. (1992)
Renfrew and Geary (1973)
Median
† Persistence
Persistence
Age range
(%)
(years;months)
46/138
38
38
3;0–7;0
5/9
25/25
22/705
4/16
23/1027
10/30
6/36
119/321
61/321
23/321
100
54
65
0
78
40
67
82
73
50
66
4;7–7;0
2;2–3;0
2;0–3;0
2;6–5;6
3;0–7;0
1;10–3;0
2;0–3;0
1;0–2;0
0;10–1;10
0;10–1;10
60/60
24/52
150/150
22
50
54
50
6;6–11;6
4;10–33;0
5;0–5;6
for the longest of three periods within the same study.
Table 6. Median persistence for receptive language only, expressive language only and
expressive and receptive language
Type of delay
(no. of studies)
Expressive and receptive
language (6)
Expressive language only (5)
Receptive language only (1)
Median persistence
(%)
Range of reported
persistence median
(%)
Age range
(years;months)
75.6
26–100
0;10–7;0
40
8.7
0–54
n/a
0;10–7;0
3;0-7;0
expressive and receptive delays are given in table 6, and the corresponding individual
studies are listed with persistence estimates in table 7. Median persistence is derived
from the single quoted Ž gure for each sample of children with the study concerned.
176
J. Law et al.
Table 7. Persistence for individual studies: receptive language only, expressive language only
and expressive/receptive language
Cases/sample size
Persistence
(%)
Age range
(years;months)
Expressive/receptive language
Richman et al. (1982)
Ward (1992)
Ward (1992)
Hall et al. (1993)
Klee et al. (1998)
Silva et al. (1983)†
Median
22/705
119/321
61/321
5/9
6/36
23/1027
65
82
73
100
67
78.2
75.6
2;0–3;0
1;0–2;0
0;10–1;10
4;7–7;0
2;0–3;0
3;0–7;0
Expressive language
Rescorla and Schwartz (1990)‡
Thal and Tobias (1992)
Scarborough and Dobrich (1990)
Silva et al. (1983)
Ward (1992)
Median
25/25
10/30
4/16
21/1027
23/321
54
40
0
28.6
50
40
2;2–3;0
1;10–3;0
2;6–5;6
3;0–7;0
0;10–1;10
23/1027
8.7
3;0–7;0
Receptive language
Silva et al. (1983)†
† Persistence
‡ Median
for the longest of three periods within the same study.
between two expressive scales reported.
To give some indication of the populations reported in tables 5 and 7, the total
number of clinical cases relative to the size of the sample is expressed as case/
sample size. The population could have been made up entirely of cases or could
have been made up of a larger population sample from which a clinical subgroup
was picked out.
Speech and language delay
Only one very early study combined speech and language delay (Fiedler et al. 1971)
in following children between 3 and 7 years of age. The relatively high Ž gure for
spontaneous improvement over the 4 years of the study needs to be contrasted
with the much higher level of persistence over the Ž rst year (87%). Of interest here
is that of the control group 29% developed problems by 4 years and 8% had minor
speech problems at 7 years. Unfortunately beyond knowing that the children failed
a screening measure at 3 years there is little in this study about the initial presenting
symptoms of the children concerned.
Language delay only
As with the prevalence data this group represents the largest single source of data.
Of the 10 studies, the range is extreme in this group although it is apparent that
taking out the studies with the smallest samples would also reduces this range
considerably. Thus, if studies with samples below 30 are removed the range shifts
to 50–82%, although the median only increases marginally (to 70%). Ward (1992)
followed infants from a mean age of 1–2 years of age (tables 5 and 7). The sample
Prevalence and natural history of primary speech and language delay
177
is divided into three groups, being expressive and receptive delay with listening
diYculties, expressive and receptive delay without listening diYculties, and expressive delay alone. There is a diVerential outcome for the three groups: while 82% of
the Ž rst group continues to show language delay, the Ž gures are 73 and 50%
respectively for groups 2 and 3. Of interest is the fact that the type of presenting
delay changed in some cases from a combined expressive-receptive delay to one of
expressive delay alone.
Speech delay only
Speech problems may appear less persistent than language problems, in that Bralley
and Stoudt (1977) report up to 78% of articulation errors resolving. However, the
long-term data from Felsenfeld et al. (1992) suggest that underlying language diYculties may continue for children originally identiŽ ed as having speech delay. Also,
there is a relatively small body of evidence from follow-up studies of children
treated for speech problems (and thus not represented in the present review) to
suggest that literacy skills are at risk even after resolution of speech delay (Stackhouse
and Snowling 1992).
The studies looking at speech development give a broad span of 6 months, 5
and 28 years follow-up, all starting with subjects initially ~ 5 years of age. Over 6
months Renfrew and Geary (1973) showed that 54% of speech cases persist, while
Bralley and Stoudt (1977) reported nearly 22% of speech problems persisting over
5 years. Felsenfeld et al. (1992) followed some of a longitudinal cohort (begun in
1960) to track adulthood outcomes. In this study children deŽ nitely had no therapy
until third grade (USA schooling, ~ 8 years); there is no account in Felsenfeld
et al.’s paper of any later therapy received. Felsenfeld et al. found 50% of children
with residual speech problems, as assessed by sentence level tests of articulation.
Language measures in adulthood also showed deŽ cits relative to controls, even
though the children were originally identiŽ ed as speech delayed. More broadly, nonverbal reasoning and personality scores in adulthood were not signiŽ cantly diVerent
between the group originally speech delayed and the control group. It should be
noted here that the controls were identiŽ ed in adulthood and were not matched to
those adults who had originally presented as speech delayed.
Language only—expressive and receptive delay
This group of studies corresponds closely to that reported in table 5 once the
smaller studies focusing on expressive delays have been reassigned, the only diVerences being that Hall (1996) remains in and the expressive group from the Ward
(1992) study is taken out. The result is that the median changes little. The Ward
data are made up of two separate groups of children both with expressive/receptive
diYculties, but one with and one without concomitant listening diYculties. In one
study 65% of delays persisted to age 4 (Richman et al. 1982) and 38% of the same
group was experiencing cognitive deŽ cits at 8 years. Of particular interest is Silva
et al.’s data (1983), which show that those children presenting with a language delay
are a  uctuating group; some children failed at each of the three language assessment
points and some failed at only one or two assessment points. There is, however,
the possibility of test-retest error contributing to this eVect. The more stable
178
J. Law et al.
subgroup comprised those children with generalized language problems aVecting
both receptive and expressive skills.
Language only—expressive delay only
Again there was a considerable disparity between the samples adopted. Two were
taken from relatively large-scale population studies (Silva et al. 1983, Ward 1992)
and the other three were recruited speciŽ cally for the study by notices in paediatricians oYces or local newspapers. Some indicated precise criteria for expressive
language delay (Rescorla and Schwartz 1990). Otherwise we were looking to identify
late talkers on the basis of expressive measures but monitored receptive language
and other skills (Scarborough and Dobrich 1990, Thal et al. 1991, Thal and Tobias
1992). The picture that emerges is one of relatively high spontaneous remission.
Interestingly, where receptive skills are monitored there is some indication that
expressive skills tend to improve but that there is little change in receptive skills
(Scarborough and Dobrich 1990). Thal and Tobias (1992) noted a better outcome
for those children with normal receptive skills and who use gesture to compensate
for their lack of expressive output.
Additional outcomes for areas other than spoken language are given by three
studies (Scarborough and Dobrich 1990, Richman et al. 1982, Silva et al. 1987) (table
5 and 7). These all point to reduced reading skills at age 7 or 8 years among those
with earlier language delay (whether or not that oral language delay has resolved).
Language only—receptive delay only
Only one study sought to identify a group of children with receptive language delay
in the absence of expressive delays (Silva et al. 1982). Although it did prove possible
to identify such a group the fact that only two of 23 children continued to have
problems rather suggests that such children are not at obvious risk for persistent
language diYculties. Nevertheless 46% of this group did go on to have reduced IQ
and reading performance at age 7. This also suggests that this is a lower risk group
than the expressive delay only group if language measures are the target outcome
but of comparable risk when other areas of development are taken into consideration. The risk of persistent problems would appear to be much lower than that for
the expressive/receptive delay group.
Summary of major Ž ndings
Inevitably such a review is a retrospective exercise and it is only possible to use the
classiŽ cation framework adopted by the authors concerned. Combining data across
studies can be problematic because of potential incompatibility. Nevertheless there
are major diVerences in the range of persistence reported. In the main the Ž gures
for speech delay appear to be more variable than those for language delay. Equally
there is considerable variability in the persistence of speech plus language delay in
untreated samples. The highest level of persistence is that for expressive and
receptive language delay, in turn suggesting that it is this group which is recognized
and best deŽ ned in terms of its case status.
Prevalence and natural history of primary speech and language delay
179
Discussion
At one level there does seem to be a measure of consensus emerging regarding the
prevalence of speech and language delay. The median of 5.95% quoted here
corresponds well with the 5.59% that was the earliest estimate quoted for speech
impairment (Blanton 1916) and with the 6% found by Provonost (NINDS 1969)
in a large-scale study (87 288 subjects) identifying speech and language impairments
in New England (both cited in Healey et al. 1981). Neither of these works was
included in the present synthesis for methodological reasons. Of course reliability
does not necessarily imply validity and it is unclear whether the deŽ nitions adopted
are comparable from a vertical (severity of delay) or a horizontal (range of included
measures) perspective. However, this level of convergence suggests that some
agreement may be emerging as to what level of diYculty should be considered a
clinical problem. In few of the studies reviewed was any attempt to monitor
explicitly persistent schooling diYculties. Nevertheless it might reasonably be argued
that the level set corresponds to a clinical understanding about which children are
most ‘at risk’.
These estimates suggest that speech and language delay in early childhood is a
common problem which can have important implications for the child. Whether
such delay is, in fact, a suYciently important health issue to merit population
screening is addressed elsewhere (Law et al. 1998). Even a moderate or mild delay
may cause appreciable concern to parents and other carers. For a high prevalence
condition such as speech and language delay the priority in terms of the screening
process is to prevent the identiŽ cation of false positives (maximize speciŽ city) while
maximizing the proportion of true positives (sensitivity). Acceptable levels of
accuracy of such measures are discussed in Law et al. (1998) but their validity will,
in the end, depend on the relative costs of over and under identiŽ cation. There are
also social and economic costs associated with diVerent outcomes but these have
yet to be explored. The natural history data suggest that a predictive element needs
to be built into any such programme. To date no such model exists.
The present review indicates that at least two common beliefs are not based on
evidence from existing prevalence data. There is little evidence to suggest declining
prevalence across the 0–16-year age range, with the possible exception of a sharp
drop in expressive delays after 2 years, and there are no data to suggest that the
prevalence has increased over the past 30 years. This apparent stability appears to
contradict the current understanding of the progression of developmental language
delays. In particular, the natural history data suggest that for many children (~ 60%
of children with early expressive delays, perhaps 25% with expressive/receptive
delays) diYculties resolve. The explanation for the stability comes from the relationship between prevalence and incidence. If the increase in the number of cases
corresponds to the decrease resulting from spontaneous remission the prevalence
will remain static. Clearly the key to good prevalence data is longitudinal rather
than cross-sectional data. There is no evidence which would suggest that there is a
real increase in cases during the 30 years covered by the review (1967–97) despite
the fact that many report increased rates of referral and educational placement for
these children particularly in the early school years ( Jowett and Evans 1996, Reid
et al. 1996, SOEID 1996). This suggests that the estimation of prevalence and the
demands made on services are not necessarily equivalent. Indeed, referral is likely
to be a function of the level of service provision and cannot be equated with
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J. Law et al.
prevalence. Only with a greater understanding of the longitudinal picture of cases
can prevalence be established.
The present data set included studies where replicable criteria for case status
were provided. In the majority of cases this involved the application of standardized
assessment measures. The advantage of this approach is that the resulting data are
easier to compare with those from other studies. The disadvantage is that there is
an implicit circularity in setting a cut-oV on a measure that has been developed on
a normal population. Identical cut-oVs on standardized measures applied at diVerent
ages will necessarily result in the same proportion of the population being identiŽed. Any
discrepancy between the proportion of the population predicted from the cut-oV
adopted and that found in the prevalence study can better be explained by the
potential diVerences between the two populations than by diVerences in the rate of
true cases in the population studied. Cut-oVs tend to follow psychometric convention without any direct attempt to link them with clinical judgement. Clinical
judgement of case status may also be problematic because it is likely to relate to
availability of services and to the anticipated response to therapy but, as Tomblin
et al. (1997) found, it is likely to be closely related to psychometric norms and an
explicit link between the two may result in higher clinical validity. Stronger support
for prevalence estimates would be indicated if the rates derived by clinical judgement
of case status showed agreement with those rates derived using conventional
cut-oV scores.
The review was charged with the identiŽ cation of gaps in the literature. Apart
from the lack of clarity regarding the level of case status there is one major omission
in the data. The one important group that has been almost completely ignored in
the available prevalence literature related to speech and language disorders is the
child with pragmatic impairments. It is true that this group has only relatively
recently been identiŽ ed as communication impaired and it is true that many such
children would be subsumed into the autistic continuum. However, given the
intransigent nature of many of the problems experienced by these children (Rutter
et al. 1992), it is important to know whether they have been incorporated into the
other Ž gures or omitted as a distinct non-linguistic category.
The evidence suggests that our understanding of the natural history of speech
and language delay, and thus the deŽ nition, of case status remains something of an
imprecise science at present. Improving prediction should help resolve issues of
case deŽ nition by demonstrating what would happen if the children were not
treated. Good and poor outcome groups could then be traced back to the point at
which they were initially identiŽ ed and relative risk assigned to individual proŽ les.
Furthermore, it should be possible to establish what has been referred to as the
‘critical point’ in the progress of the disability before which therapy is either more
eVective or easier to apply than afterwards (Sackett et al. 1991). This review conŽ rms
the conclusion reached by a number of other studies that children with language
delays which incorporate both expressive and receptive skills present the clearest
picture. These children are likely to Ž nd it diYcult to process incoming language,
to initiate communication with others and to formulate their responses appropriately. Accordingly they are less likely to compensate for their diYculties and are
most likely to Ž nd diYculty in coping with the demands of school.
Many of these studies report persistence over comparatively short periods.
While this may be interesting in its own right it does not help address the more
central question of whether speech and language delays are likely to have a lasting
Prevalence and natural history of primary speech and language delay
181
impact on the child. If the condition is broadly benign it might be anticipated that
persistence would decline the longer the follow-up period. Alternatively it may be
that the condition left untreated leads to a wide range of social and educational
sequelae. These issues were not fully explored in the present synthesis but are highly
relevant to the area of speech and language delay. Whether these oral language
delays have resolved, multiple educational and social diYculties are noted for
children who had earlier speech/language delays. Of early expressive language
delayed children, 41–75% showed reading problems at age 8 years. This Ž nding is
conŽ rmed by the larger body of literature from follow-up studies of children who
may have received intervention for speech and language delay and who were
subsequently identiŽ ed as being dyslexic (e.g. Catts 1991).
Indeed there is also the issue of the prognosis for children with speech and
language delays who have received intervention. Follow-up studies tend to paint a
more negative picture of outcomes for children whose diYculties do not resolve
by school entry (Petrie 1975, Bishop and Edmundson 1987, Cook et al. 1988,
Huntley et al. 1988, Urwin et al. 1988, Haynes and Naidoo 1991, Davison and
Howlin 1997). The Ž ndings reveal that in addition to continuing problems in verbal
language, reading, spelling and other aspects of educational attainment can also be
aVected (Aram and Nation 1980, Stark et al. 1984, Bishop and Adams 1990, Catts
1991, Morris-Friehe and Sanger 1994, Tallal et al. 1997) together with behaviour
and other aspects of psycho-social adjustment (Baker and Cantwell 1987, Silva et al.
1987, Beitchman et al. 1989, 1996). While the problems are more marked for those
children whose language diYculties are associated with low intellectual ability or
those with both receptive and productive language diYculties, those with primary
delay can also experience marked long-term diYculties which may persist into
adulthood (Haynes and Naidoo 1991). How does this rather more negative picture
square with the natural history data reported above? The main reason for this
diVerence is that these follow-up studies in the main report data on children who
are already conŽ rmed ‘clinical’ cases and are, therefore, already in the highest risk
group. By including only the most severe cases they may overestimate the negative
sequelae of the population of speech and language delayed children as a whole.
Given the level of persistence in the treated groups it might be more appropriate
to argue that spontaneous improvement in this group is no longer the central issue.
Rather the outcome should be how well the children can cope with the disabilities
associated with their impairments.
The nature of the concept of natural history needs to be addressed. While it
may be technically possible to provide no input at all, in reality the needs of the
children contribute to the world around them. They receive help from parents,
from teachers and nursery staV, which may re ect those needs. Intervention of this
type may not be comparable to specialist speech and language input but it is diYcult
to say that it has no eVect. The term ‘natural history’ is derived from less contextdependent interventions. Thus, a drug regimen may be said to be in place or not.
If it is not, it may be possible to follow the course of the disease. However, it is
not clear that the same is true of speech and language delays especially in children
who, as a group, may be paid particular attention by all those around them by
virtue of those very diYculties. A second diYculty in interpretation is the ethical
implications of withholding intervention in order to carry out such studies. While
this may have been true when no support services were available, for example for
the children in the Dunedin study (Silva et al. 1987), this is now not the case for
182
J. Law et al.
most children in the English-speaking world at least. On what grounds would it be
reasonable to withhold intervention for the sort of length of time such that it
would be meaningful to ascertain natural history? It has been suggested that 3 years
is an appropriate period over which to measure change (Aylward 1988). To a certain
extent the answer to this question depends on what is known about intervention.
On the one hand if one cannot show that intervention is eVective it may be that
natural history is a reasonable concept. On the other hand if beneŽ ts can be
demonstrated then the case for withholding treatment over the long-term recedes.
The intervention data reviewed in the present synthesis (Boyle at al. 1999) has
suggested that there is a good case for assuming that intervention can be shown to
be beneŽ cial although the data set remains comparatively small.
This has led some commentators to suggest that a distinction be drawn between
routine intervention and specialist intervention (Shriberg 1994, Shriberg et al. 1994).
Community provision of speech and language therapy could constitute natural
history in the sense that children receive other services within the healthcare system,
for example the services of a family doctor or a community nurse. Such services
are often of a lower level of intensity than specialist provision and might reasonably
be construed as the normal activities of social or health services. However, such
an approach does not really help overcome the essentially rather arbitrary distinctions that are drawn between diVerent levels of input. Where does specialist
intervention begin—once children have been referred to specialist provision? The
move towards integrating children into their local community schools may mean
that fewer children would be receiving the kind of speciŽ c intervention that would
preclude them from involvement in natural history studies of the kind reviewed here.
A model of predictive risk
Interpretation of the data is dogged by an uncertainty as to which children are truly
cases in the sense that without intervention they would necessarily go on to
experience problems associated with early speech and language delay in school and
beyond. This issue is complicated further by social constructs of ‘normality’ and
the diYculty in establishing a true ‘gold standard assessment’ with a theoretically
determined deŽ nition of case status. An alternative approach is to move away from
the model adopted in the above studies and towards a model of predicted risk.
This would involve identifying the key predictive factors of subsequent case status
and to build them into a model that identiŽ es an ‘at risk’ group. This is a relatively
new concept in the Ž eld of speech and language therapy, although see Tomblin
et al. (1997) for some early work in quantifying that risk with relation to SLI. Much
depends on the outcome measures adopted. It seems likely that speech and language
measures alone will not provide a meaningful outcome and literacy and other
educational measures will need to be built into such a model ( Beitchman et al. 1996,
Drillien et al. 1988, Glascoe 1999). Such an approach would circumvent the need
to carry out separate prevalence studies in bilingual or disadvantaged populations
but it would mean gathering data, which included representative populations, rather
than attempting to model outcomes on a discrete clinical population as is often the
case. It would not be a matter of reporting, as most authors have already done,
that males are over-represented in this group of children but of establishing to what
extent gender is a risk factor in diVerent subcategories of the population. The
advantage of such an approach is that it could make use of the available genetic
Prevalence and natural history of primary speech and language delay
183
( Bishop et al. 1995), and intervention data (Boyle et al. 1999a, b) along with data
regarding co-morbidity such as behaviour (Stevenson 1996) as it becomes available
to modify and improve the model. Similarly such a model could begin to test the
Ž ndings of contributory factors identiŽ ed in other studies. EVectively a model
based on risk would be one that embraced the variance in the population.
Early attempts to model the progression of speech and language disorder have
relied heavily on regression analysis (Schery 1985) and have often included essentially
normal middle class children in their samples (Bee et al. 1982). However, there is a
need to look at the capacity of independent variables to discriminate clinical
outcomes. To date there have been a great many studies examining the association
between other factors and language delay/disorder (Lassman et al. 1980). But with
three exceptions little attention had been paid to the risk attached to these associations (Drillien et al. 1988, Tomblin et al. 1997, Paul and Fountain 1999). Tomblin
et al., in a retrospective study, adopted the odds ratio metric, a means of ascertaining
the level of association between the occurrence of a particular condition and a
particular risk factor. The odds of having an exposure to a given risk among clinical
cases is contrasted with the odds of having it in the normal population. They
calculated the odds ratios associated with case status of SLI at 5 years and found
that paternal rather than maternal history of speech and language diYculties and
paternal smoking during foetal development, were particularly associated with
speciŽ c language diYculties. Conversely breast-feeding and length of breast-feeding
functioned as a protective factor eVectively reducing the risk of SLI. Paul and
Fountain (1999) examined a small prospective cohort of thirty six children with
speciŽ c expressive language delays identiŽ ed at 20–34 months and followed up to
7 years of age. Using discriminant function analysis, they found that the best
predictors of outcome were socio-economic status, parent report of adaptive behaviour and gross motor skills. Although case status in these studies was comparable
(10th centile) the measures diVered as did the variables put into the analyses which
were in themselves based upon diVerent statistics. In neither case was the level of
intervention factored into the analysis.
It is also possible to look at speech and language delays as markers for other
conditions (Whitehurst et al. 1991). Drillien et al. (1988), for example, used early
language performance as predictors of subsequent language and educational development. They examined the relationship between developmental screening results in
Ž ve developmental areas and their association with subsequent educational and
language performance. They reported signiŽ cant odds ratios of 1.60 (CI 1.12–2.27)
to 2.44 (1.5–3.97) for the association between all aspects of developmental screening
and subsequent expressive language diYculties but found much weaker associations
between early expressive language diYculties and subsequent school performance.
This corresponds well with the relatively low level of persistence of expressive
diYculties reported in the present review. Each of these associations was calculated
on the basis of the individual child’s performance at developmental screening checks
between 20 weeks and 5 years. Like Paul and Fountain they emphasized the saliency
of adaptive behaviour and motor skills in the predictive model.
The aim of such analyses would be to provide the clinician with the tools
to say that, given the following presenting symptoms, and predisposing and
ameliorating factors the following outcome could be predicted given the following
intervention. For example, it might be possible to say that a child with an expressive
delay has an even chance of developing persistent eVects either in language or in
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J. Law et al.
other skills. These odds may lengthen if negative social interaction and medical
factors are included but may shorten if short-term intervention of known impact
is added into the equation. For example, Boyle et al. (1999a, b) indicate a mean
eVect size of the order of 1 SD for relative short-term ‘dosage’ interventions for
expressive language delays in the pre-school years. Conversely, it might be possible
to say that a child with an expressive/receptive diYculty may have a seventy Ž ve
percent chance of developing associated diYculties, for example a risk which may
be increased if other contributory factors are added but which may not be alleviated
with short-term intervention. Finally there may be diVerential eVects of impairments
and environmental eVects across time. For example, there is some suggestion that
for children with motor diYculties the motor skills serve as good predictors in the
short-term but the environment plays as a more substantial role as the child gets
older (Cohen et al. 1986). The role of social class is clearly one that needs to be
examined with care. Across a population it is likely to operate as a proxy variable
accounting for a considerable proportion of the variance. However, in doing so it
is likely to mask the eVects of other components in any analysis, components,
which may be more amenable to modiŽ cation at the level of the individual.
Reviews of the literature often end with a call for more research. The conclusion
drawn here is that while health service providers may wish for measures of prevalence upon which to base services it would be premature to call for more prevalence
data. More population estimates may be warranted particularly in relation to the
needs of local populations but it is the risk not the single prevalence estimate
towards which progress most needs to be made. There is a need for large-scale
cohort studies to tease out the relationship between the component parts of the
equation across time and to establish a better understanding of the incidence of
speech and language delays. More attention needs to be paid to the risk and
protective factors and the extent to which they are in uenced by confounding
variables and their interactions.
Conclusions
This paper has demonstrated the close association between prevalence and natural
history. Appreciation of one cannot follow without a good understanding of the
other. Ultimately the issue revolves around the concepts of prediction and relative
risk. Such issues can only be properly dealt with using longitudinal population data
and with clinical validation of psychometric classiŽ cation.
Acknowledgements
The NHS Centre commissioned this systematic review for Reviews and
Dissemination at the University of York. F. H. received support from the
Department of Clinical Communication Studies, City University, London, in the
latter part of this project. The authors also thank Christopher Norris, Biomedical
Research Indexing, and two anonymous reviewers of the paper.
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