Reliability, Sensitivity to Change, and
Responsiveness of the Peabody Developmental
Motor Scales−Second Edition for Children With
Cerebral Palsy
Hsiang-Hui Wang, Hua-Fang Liao and Ching-Lin Hsieh
PHYS THER. 2006; 86:1351-1359.
doi: 10.2522/ptj.20050259
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Research Report
䢇
Reliability, Sensitivity to Change, and
Responsiveness of the Peabody
Developmental Motor Scales–Second
Edition for Children With
Cerebral Palsy
ўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўў
Background and Purpose. The psychometric properties of the Peabody Developmental Motor Scales–Second Edition (PDMS-2), a revised motor test to
assess both gross motor and fine motor composites in children with cerebral
palsy (CP), are largely unknown. The purpose of this study was to examine the
test-retest reliability and the responsiveness of the PDMS-2 for children with
CP. Subjects. A sample of 32 children who had CP (age⫽27– 64 months) and
who received intervention participated in this study. Methods. The PDMS-2
was administered to each child 3 times (at the beginning of the study, at 1
week, and at 3 months later) by a physical therapist. The agreement between
the first 2 measurements was used to examine the reliability. The change
between the first and the third measurements was used to examine the
responsiveness. Results. The composite scores on the PDMS-2 had good
test-retest reliability (intraclass correlation coefficient⫽.88 –1.00). The
sensitivity-to-change coefficients ranged from 1.6 to 2.1, and the responsiveness coefficients ranged from 1.7 to 2.3. Discussion and Conclusion. Our
results provide strong evidence that the 3 composites of the PDMS-2 had high
test-retest reliability and acceptable responsiveness. The PDMS-2 can be used
as an evaluative motor measure for children with CP and aged 2 to 5 years.
[Wang HH, Liao HF, Hsieh CL. Reliability, sensitivity to change, and
responsiveness of the Peabody Developmental Motor Scales–Second Edition
for children with cerebral palsy. Phys Ther. 2006;86:1351–1359.]
Key Words: Cerebral palsy, Measurement: applied, Measurement: basic theory and science, Reliability of
results.
ўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўў
Hsiang-Hui Wang, Hua-Fang Liao, Ching-Lin Hsieh
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Physical Therapy . Volume 86
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1351
D
uring the past 20 years, physical therapists
have had considerable interest in the development and evaluation of health status outcome measures.1 Outcome measures are used
by researchers and clinicians to assess changes in
patients’ abilities before and after health care to promote the accountability of health care services.2 Outcome measures must have the psychometric properties
of reliability and responsiveness.3–5 The low intrasubject
variation in stable subjects reflects the test-retest reliability of a measure.3 Only measures with high test-retest
reliability can detect real change and reduce the bias
caused by measurement error. The responsiveness of a
measure is defined as the ability to assess clinically
important change over time.6 Thus, evidence supporting
the test-retest reliability and responsiveness of an outcome measure must be established before its use in
research or clinical settings.
Cerebral palsy (CP) describes a group of disorders of the
development of movement and posture, causing activity
limitation, that are attributed to nonprogressive disturbances that occurred in the developing fetal or infant
brain.7 To evaluate the effectiveness of treatment for the
motor domain, clinicians need a motor evaluative tool.
The Gross Motor Function Measure (GMFM)8 and the
Peabody Developmental Motor Scales (PDMS)9 are the 2
most well-known motor instruments for children with
CP. However, the GMFM measures the gross motor
domain only.8 For measurement of the fine motor
domain, the GMFM is inadequate as an evaluative tool.
A previous responsiveness study with the gross motor
(GM) composite of the PDMS (PDMS-GM) for infants
with CP showed that the PDMS-GM had limitations when
used as an evaluative measure for infants with CP.10 The
PDMS has been revised to the Peabody Developmental
Motor Scales–Second Edition (PDMS-2), with new
norms, revised testing materials, more precise scoring
criteria, and more information on norm samples.11 Each
item of the PDMS-2 was evaluated with both conventional item analyses and modern differential item functioning analyses to select the appropriate items. New
normative data on the PDMS-2 were collected through
1997 and 1998 for a sample of 2,003 children residing in
the United States and Canada; not only children without
disabilities but also 10% of children with various types of
disabilities were included in the sample. There are also
more reliability and validity data for the PDMS-2 than for
the PDMS.11 Therefore, the PDMS-2 is potentially appropriate for investigating the progress of the gross and fine
motor domains for children with CP because it assesses
both GM and fine motor (FM) composites and incorporates both quantitative and qualitative rating criteria.
The concurrent validity studies of the standard scores on
the PDMS-2 showed high correlations with the PDMS or
the Mullen Scales of Early Learning: AGS Edition in the
GM or the FM composite (r ⫽.80 –.91) for children for
whom detailed information on health conditions was not
available.11 For children with developmental delays,
although the developmental quotients (DQ) of the
PDMS-2 were significantly correlated with the Bayley
Scales of Infant Development–Second Edition, the classification agreement between these 2 tests was poor.12
The construct validity of the PDMS-2 was established by
confirmatory factor analyses, and the results showed that
the GM and the FM composites are 2 separate constructs
within general movement. Another construct validity
study of the PDMS-2 demonstrated high correlations
HH Wang, RPT, MSc, is Pediatric Physical Therapist, Country Hospital, Taipei, Taiwan.
HF Liao, RPT, MPH, is Associate Professor, School and Graduate Institute of Physical Therapy, College of Medicine, National Taiwan University,
No. 17, Syujhou Rd, Taipei City, Taiwan, Republic of China. Address all correspondence to Ms Liao at: hfliao@ntu.edu.tw.
CL Hsieh, OTR, PhD, is Professor, School of Occupational Therapy, College of Medicine, National Taiwan University.
All authors provided writing and data analysis. Ms Wang and Ms Liao provided concept/idea/research design and data collection. Ms Wang
provided coordination of institutes and subjects. Ms Liao provided project management and fund procurement. The authors thank the following
rehabilitation departments and the therapists of the institutes for assisting with data collection: National Taiwan University Hospital; Lo-Tung
Pohai Hospital; Kee-Lung General Hospital; Department of Health, Executive Yuan, Taiwan, Republic of China; Buddhist Tzu Chi General
Hospital; Cathay General Hospital; Cardinal Tien Hospital; Country Hospital; and 2 developmental centers, Syin-Lu and Di-Yi. They also thank the
caregivers and children who participated in this study and Dr Jeng-Yi Shien for her valuable help.
This study was reviewed and approved by the Ethics Committee of National Taiwan University Hospital.
This study was supported by the Department of Health, Executive Yuan, Taiwan, Republic of China (DOH 92TD1016).
This article was received August 18, 2005, and was accepted May 2, 2006.
DOI: 10.2522/ptj.20050259
1352 . Wang et al
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between age and subtest raw scores.11 One recent study13
showed that the overall diagnostic accuracy of the
PDMS-2 was high, with an area under the receiver
operating characteristic curve of 0.98 for children with
motor disabilities. These results indicate that clinicians
could diagnose motor disabilities correctly 98% of the
time with the test results of the PDMS-2.14 One of the
purposes of the PDMS-2 is to evaluate a child’s progress
after intervention.11 However, norm-referenced motor
assessments should not be used as evaluative measures
until they are validated to have acceptable responsiveness for children with motor dysfunction.4 Because the
responsiveness of the PDMS-2 for children with CP is still
unknown, one purpose of this study was to investigate
the responsiveness of the PDMS-2 for children with CP.
The reliability of PDMS-2 scores was investigated by the
test developers. In their study, 3 types of error variance—
internal consistency, test-retest reliability, and interscorer reliability—were investigated. All of the reliability
coefficients for 3 composites and 6 subtests of the
PDMS-2 (Cronbach ␣⫽.89 –.97, test-retest r ⫽.82–.93,
and interscorer r ⫽.96 –.99) showed that the PDMS-2 is a
reliable tool for the assessment of motor development in
children.11 However, only children without disabilities
were recruited for that reliability study. Because the
reliability levels may vary for different populations,15 the
reliability of PDMS-2 scores for children with CP needs
further investigation. The test-retest reliability coefficients are often thought of as stability coefficients;
however, they do not reveal how much variability should
be expected on the basis of measurement error.16 Thus,
for estimating the confidence intervals of test scores, the
standard error of measurement (SEM) of the PDMS-2
for children with CP needs to be calculated. Therefore,
the other purpose of this study was to examine the
test-retest reliability and SEM of the PDMS-2 for children
with CP.
Method
Participants
Previous studies17,18 showed that the score change of
motor tests was affected by age and CP severity in
children with CP. There was an age and severity interaction on the amount of change in the GMFM-88 scores.17
Children who were young and had mild CP demonstrated greater change in the GMFM-66 scores than did
children who were older and had more severe CP over a
period of time.18 To make the age (⬍48 mo or ⱖ48 mo)
and severity (mild or severe) levels evenly distributed in
the present study sample, a quota sample of 32 children
with CP was recruited.
Children were recruited from 2 developmental centers
and 7 hospitals in the northern and eastern areas of
Taiwan. To be eligible to participate in the study,
children had to meet the following criteria: a confirmed
medical diagnosis of CP from the attending pediatrician,
age ranging from 24 to 65 months at the first evaluation,
receiving physical therapy or occupational therapy at
least twice per month during the study period, and
written informed consent of the caregivers or guardians.
The underpinnings of the therapy approaches that the
children received were based on the patient/client management model,19 the International Classification of Functioning, Disability and Health model,20 the family-centered
approach,21 and motor learning strategies.22 The International Classification of Functioning, Disability and Health
model can be used to evaluate possible influencing
factors (impairment, environmental, and personal factors) for motor disabilities and mobility for children and
then to set treatment plans and goals. The exclusion
criteria were as follows: a medical problem that might
prevent participation in therapy programs and progressive neurological disorders or medical conditions for
which progress in motor development would not be
expected over a 3-month period. In epidemiology studies, ages ranging from 2 to 10 years have been chosen as
the age of ascertainment for CP diagnosis.23 Furthermore, the upper limit of the suitable age for testing
children with the PDMS-2 is 71 months. Therefore, we
set the minimum age for children at 24 months and the
maximum age at 65 months.
The severity of CP in the children with CP was measured
according to the Gross Motor Function Classification
System (GMFCS)24 and was rated by the physical therapists treating those children and confirmed by one
senior physical therapist before the PDMS-2 assessment.
In this study, children at GMFCS levels I and II were
classified as having mild CP, and those at GMFCS levels
III to V were considered to have severe CP.
The mean age, body height, body weight, CP severity
level, and sex of the children at the first evaluation are
shown in Table 1. Their ages ranged from 27 to 64
months. The clinical types of CP in these children were
spastic hemiparesis (n⫽5), spastic diplegia (n⫽14), spastic triplegia (n⫽4), spastic quadriplegia (n⫽6), and
ataxia (n⫽3). The ages (X⫾SD) of the fathers and
mothers were 37⫾5.8 and 34⫾5.6 years, respectively.
The education levels of the fathers and mothers, respectively, were graduate school (n⫽2 and 0), university or
college (n⫽11 and 9), senior high school (n⫽11 and
17), junior high school (n⫽4 and 3), and primary school
(n⫽2 and 2). The occupations of the fathers and mothers, respectively, were professional or central administrators (n⫽6 and 1), semiprofessional workers (n⫽10 and
6), technical workers (n⫽10 and 2), and semitechnical
or nontechnical workers (n⫽4 and 22).25 For 1 child,
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Table 1.
Demographic Data for Children With Cerebral Palsy (CP)
XⴞSD
Younger
Older
Parameter
Mild CP
Severe CP
Mild CP
Age at first test (mo)
34.0⫾5.9
39.8⫾2.8
54.5⫾7.0
54.6⫾6.4
Age at start of intervention
(mo)
13.1⫾10.5
10.6⫾4.8
16.4⫾10.0
23.0⫾9.8
Body height (cm)
88.8⫾5.8
92⫾8.1
102.9⫾4.8
97.0⫾4.0
Body weight (kg)
13.1⫾2.4
13.9⫾2.9
16.6⫾2.7
13.6⫾2.1
No. of boys/girls
5/3
7/1
6/2
information on the social or economic status of his
family was not available.
Study Design
The single-group design method for examining both
reliability and responsiveness was used in this study.26
One physical therapist assessed each child 3 times; the
period between the first and second measurements was
about 1 week, and the duration between the first and
third measurements was 3 months. The agreement
between the first 2 measurements was used to examine
test-retest reliability. The change between the first and
third measurements was used to examine responsiveness.
A previous study27 indicated that responsiveness studies
involving children with CP need to be at least of 3
months’ duration. In line with this finding, we set the
duration between the first and third measurements at 3
months.
Usually a measure must be sensitive to change before it
can be responsive.6 Sensitivity to change is the capacity of
a measure to assess change over time.6 Thus, 2 types of
change indexes were used in this study; one was the
sensitivity-to-change coefficient, and the other was the
responsiveness coefficient. The responsiveness coefficient can be calculated from the differences of score
change between groups of subjects who have and subjects who have not experienced “clinically important
change” on the basis of retrospective judgment.28 To
examine responsiveness in this study, a caregivers’ rating
scale to detect a retrospective global rating of change was
designed on the basis of a previous study.29
Assessment and Instruments
The PDMS-2 is a standardized, norm-referenced test.11
The GM composite of the PDMS-2 includes 151 items
from 4 subtests: reflexes, stationary, locomotion, and
object manipulation. The FM composite comprises 98
items from 2 subtests: grasping and visual-motor integra-
1354 . Wang et al
Severe CP
5/3
tion. The total motor (TM) composite
includes 249 items from all subtests.
Items of the PDMS-2 are scored with a
3-point score (0, 1, and 2); a score of 2
is assigned when the child performs the
item according to the specified item
criterion, a score of 1 indicates that the
behavior is emerging but that the criterion for successful performance is not
fully met, and a score of 0 indicates that
the child cannot or will not attempt the
item or that the attempt does not show
that the skill is emerging. Therefore,
the maximum raw scores of the subtests
are different, ranging from 16 to 198.
From the results of the raw scores on each subtest of the
PDMS-2, the standard scores and developmental age
equivalents on each subtest can be obtained from the
norms in the manual for the PDMS-2. The DQs for the
GM, FM, and TM composites then are derived by
summing the subtest standard scores and converting
them to a quotient with a mean of 100 and a standard
deviation of 15.11 Folio and Fewell11 suggested that to
make important decisions about diagnosis and placement for children, the clinician should rely primarily on
the results of composites rather than subtests. Therefore,
this study focused on composite scores only. For the 3
composites of the PDMS-2, only the raw scores, percentile scores, and DQs can be obtained from the PDMS-2
manual. In clinics, the percentile scores and DQs for the
3 composites of the PDMS-2 can be used to share the test
results with others and to identify the risk for or severity
of the motor developmental delay. Clinicians should
know the possible magnitudes of the measurement
errors of these scores. Therefore, we analyzed the testretest reliability coefficients and SEMs of the raw scores,
percentile scores, and DQs for the 3 composites.
Change indexes for measures usually were calculated
from raw scores, percentage scores, and scaled scores in
previous responsiveness studies for children.27,30,31 Percentile scores and DQs are scores adjusted by age and
are not suitable to be used as outcome indexes.27 The
PDMS-2 does not provide information on scaled scores;
therefore, only raw scores on the 3 composites of the
PDMS-2 were used to calculate change indexes in this
study.
The caregivers’ rating scale is composed of 3 items that
are closed-ended questions that ask the main caregivers’
perception about overall change in GM, FM, or TM areas
in the previous 3 months and are based on a 7-point
Likert scale (much better, better, somewhat better,
about the same, somewhat worse, worse, and much
worse). The rating scale was self-administered by the
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main caregiver of the child (usually the mother) at the
time of the third measurement. The test-retest reliability
of the caregivers’ rating scale for motor change within 1
week was analyzed by the quadratic weighted kappa
coefficient test.32 The test-retest reliability (kappa coefficient) values of the caregivers’ rating scale were .63
(GM), .43 (FM), and .54 (TM), indicating moderate
reliability.14 Because of the moderate test-retest reliability of the caregivers’ rating scale scores, we administered
the caregivers’ rating scale 2 times with a 1-week interval
to achieve more stable ratings. For calculating the
responsiveness coefficient, only children who were rated
“somewhat better,” “better,” or “much better” at both
times were classified as having clinically important
change.
Procedure
All caregivers of the children tested were informed of
the procedure and purposes of the study and signed
consent forms. The PDMS-2 assessments were administered by following the standard procedures outlined in
the test manual.11 At the third assessment, the caregivers’ rating scale was administered. All of the testing was
performed by a physical therapist (with 2 years of
working experience with children with CP) who was
familiar with the PDMS-2 and had good interrater reliability with a senior physical therapist (intraclass correlation coefficient [ICC]⫽.99 –1.00 for raw scores or DQs
for the 3 composites). Most assessments were performed
in the places at which the children received treatments
regularly. A few assessments were performed at a local
child assessment laboratory because of a lack of appropriate space for assessments at the original treatment
area. Each child received 3 assessments at the same time
during the day.
Data Analysis
In order to attain even contributions of subtests for each
composite, the raw scores on each subtest were transformed to the percentage score, as has been done for
GMFM-88 scores.17 For example, the percentage score
on the stationary subtest equaled the raw scores on the
stationary subtest divided by the maximum raw score on
the stationary subtest multiplied by 100. The percentage
score on the GM composite was the average of the
percentage scores on 4 subtests (reflexes, stationary,
locomotion, and object manipulation). The percentage
score on the FM composite was the average of the
percentage scores on 2 subtests (grasping and visualmotor). The percentage score on the TM composite was
the average of the percentage scores on all 6 subtests.
Statistical analyses were performed with SPSS (Statistical
Package for the Social Sciences) version 10.0.* Testretest reliability and change indexes were computed as
follows.
Test-retest reliability. The ICC(2,1) was used to analyze
the test-retest reliability of the raw scores, the percentage
scores, the percentile scores, and the DQs for the 3
composites between the first and second assessments.33
In general, values of ICC of less than .5 can be interpreted as indicating poor reliability, those between .5
and .75 can be interpreted as indicating moderate
reliability, and those above .75 can be interpreted as
indicating good reliability.14 The SEMs for the different
scales of the 3 composites of the PDMS-2 also were
calculated.15
Sensitivity to change. Four statistical analyses were performed to calculate the sensitivity-to-change coefficient:
the t value of the paired t test, the effect size (ES), the
standardized response mean (SRM), and the Guyatt
responsiveness index (GRI) for sensitivity to change
(GRI-S). The t value of the paired t test is used to analyze
data originating from a 1-group repeated-measures
design and concludes whether a statistically significant
change in the measures over time exists or not. The ES
is a standardized measure of change obtained by dividing the average change between initial and follow-up
measurements by the SD of the initial measurement.26 In
this study, the ES was calculated by dividing the average
change between the first and third tests by the pooled
SDs of the first and third tests. The value of the ES is
interpreted as trivial (ES of ⬍0.2), small (ES of ⱖ0.2
⬍0.5), moderate (ES of ⱖ0.5⬍0.8), or large (ES of
ⱖ0.8) according to the well-known thresholds of
Cohen.34 The SRM equals the mean change in scores
divided by the SDs of subjects’ difference scores.35
Therefore, in this study, it was calculated by dividing the
average change between the first and third tests by the
SDs of the score differences between the first and third
tests. To interpret the value of the SRM for each
composite, the ES thresholds (0.2, 0.5, and 0.8) proposed by Cohen34 were converted to SRMs according to
the correlation coefficients between the scores on the
first and third tests in this study and the formula
proposed by Middel and van Sonderen36; then, the
magnitude of the SRM was interpreted as trivial, small,
moderate, or large according to the derived values. The
GRI represents the ratio of observed change (or clinically important difference, if it is known) in a group of
subjects expected to undergo a change to the variability
in stable subjects.37 For sensitivity to change, in this
study, the GRI-S was calculated by dividing the average
change between the first and third tests by the standard
* SPSS Inc, 233 S Wacker Dr, Chicago, IL 60606.
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Table 2.
Test-Retest Reliability and Standard Error of Measurement (SEM) for Developmental
Quotients, Percentile Scores, Raw Scores, and Percentage Scores for the Gross Motor (GM),
Fine Motor (FM), and Total Motor (TM) Composites of the Peabody Developmental
Scales–Second Edition for Children With Cerebral Palsy
XⴞSD
Intraclass Correlation
Coefficient (95%
Second Test Confidence Interval)a SEM
percentile scores, from .993 to .996 for
the raw scores, and from .993 to .995
for the percentage scores (P⬍.0001).
These results indicated that the 3 composites of the PDMS-2 had good testretest reliability.
Sensitivity to Change
The mean percentage scores on the 3
Developmental quotients
composites of the PDMS-2 at the first
GM
54.0⫾9.9
54.6⫾10.4 .988 (.976–.994)
1.1
and second tests are shown in Table 2,
FM
71.7⫾17.1
73.2⫾18.9 .979 (.958–.990)
2.5
TM
57.9⫾12.5
58.9⫾13.5 .984 (.968–.992)
1.6
and those at the third test are shown in
Table 3. The percentage scores were
Percentile scores
GM
1.0⫾2.2
1.0⫾1.7
.954 (.909–.978)
0.5
significantly different between the first
FM
10.7⫾15.4
13.5⫾21.0 .919 (.840–.960)
4.4
and third tests, with t(df⫽31) values of
TM
1.8⫾3.3
2.5⫾4.9
.878 (.765–.939)
1.2
4.98 to 7.35 (P⬍.001). The ES value was
Raw scores
0.2 for all 3 composites; this value met
GM
122.4⫾45.3 124.6⫾47.2 .996 (.991–.998)
3.0
the minimum standard proposed by
FM
129.2⫾35.5 130.9⫾37.1 .993 (.985–.996)
3.0
Cohen for indicating a small change.34
TM
251.6⫾73.0 255.5⫾76.1 .996 (.992–.998)
4.7
The
correlation coefficients of the perPercentage scores
centage
scores on the GM, FM, and TM
GM
49.8⫾15.9
50.4⫾16.7 .993 (.986–.997)
1.3
composites between the first and third
FM
69.4⫾17.5
70.2⫾18.5 .995 (.989–.997)
1.3
TM
56.4⫾14.7
57.0⫾15.6 .995 (.990–.998)
1.1
tests were .978, .976, and .986, respectively. Therefore, the values of the
a
For all values, P⬍.0001, as determined with the ICC(2,1) model.
SRMs were interpreted as trivial (SRM
of ⬍1.0), small (SRM of ⱖ1.0⬍2.4),
deviation of the score differences between the first and
moderate (SRM of ⱖ2.4⬍3.8), or large (SRM of ⱖ3.8)
third tests.26
for the GM composite; trivial (SRM of ⬍0.9), small (SRM
of ⱖ1.0⬍2.3), moderate (SRM of ⱖ2.3⬍3.7), or large
Responsiveness. One of the GRIs,37 which reflects the
(SRM of ⱖ3.7) for the FM composite; and trivial (SRM
extent to which change in a measure relates to correof ⬍1.2), small (SRM of ⬎1.2⬍3.0), moderate (SRM of
sponding change in a reference measure of clinical or
ⱖ3.0⬍4.8), or large (SRM of ⱖ4.8) for the TM composhealth status,35 is referred to as the GRI for responsiveite according to previously described methods.36 The
ness (GRI-R) in this study. The GRI-R is calculated by
SRM values of the percentage scores on the PDMS-2
dividing the change in the group expected to undergo a
were 1.3 for the TM composite, indicating a small
change by the variability in a stable group.26 We calcuchange, 0.9 for the GM composite, indicating a trivial to
lated the GRI-R by dividing the mean change score
small change, and 1.0 for the FM composite, indicating
between the first and third tests for subjects classified as
a small change in children with CP. The GRI-S values
having a clinically important change on the basis of the
ranged from 1.6 to 2.1 (Tab. 3).
caregivers’ rating scales by the standard deviation of the
score differences between the first and second tests for
Responsiveness
the entire group.26
The GRI-R values for the 3 composites of the PDMS-2
ranged from 1.7 to 2.3 (Tab. 4).
Results
Discussion
The DQs for the 3 composites of the PDMS-2 for the
The results of this study are the first to confirm not only
children with CP at the initial assessment are shown in
good test-retest reliability of various scales of the PDMS-2
Table 2. All children had DQs of less than 85 for the GM
but also acceptable responsiveness of the percentage
composite and the TM composite. The means of the
scores on the 3 composites of the PDMS-2 for children
percentage scores on the GM, FM, and TM composites
with CP. These observations suggest that the PDMS-2 can
were 49.8, 69.4, and 56.4, respectively.
be used as a set of evaluative tools for children with CP.
Test-Retest Reliability
Reliability is particularly important for developmental
The test-retest reliability values and SEMs for the GM,
tests, either as a diagnostic test to evaluate the severity of
FM, and TM composites are shown in Table 2. The
developmental delay in clinics16 or as an evaluative test
test-retest reliability analyses showed ICCs ranging from
.979 to .988 for the DQs, from .878 to .954 for the
to detect the progress of a child after intervention.26
Parameter
1356 . Wang et al
First Test
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Table 3.
Sensitivity-to-Change Coefficients for Percentage Scores on the Gross Motor (GM), Fine Motor (FM), and Total Motor (TM) Composites of the
Peabody Developmental Motor Scales–Second Edition for Children With Cerebral Palsy Over a 3-Month Interval
a
Composite
XⴞSD Percentage
Score at Third Test
t Value (P)
Effect
Size
Standardized
Response Mean
GRI-Sa
GM
FM
TM
52.8⫾15.5
73.3⫾17.3
59.6⫾14.7
4.98 (⬍.001)
5.68 (⬍.001)
7.35 (⬍.001)
0.2
0.2
0.2
0.9
1.0
1.3
1.6
2.1
2.1
GRI-S⫽Guyatt responsiveness index for sensitivity to change.
Table 4.
Responsiveness Coefficients for Percentage Scores on the Gross Motor
(GM), Fine Motor (FM), and Total Motor (TM) Composites of the
Peabody Developmental Motor Scales–Second Edition for Children
With Cerebral Palsy Over a 3-Month Interval
Composite
Mean Change
Scorea
SD of
Differencesb
GRI-Rc
GM
FM
TM
3.2
4.4
3.4
1.9
1.9
1.5
1.7
2.3
2.3
a
Mean change score between the first and third tests for children rated more
than or equal to “somewhat better” (percentage score).
b
SD of differences⫽standard deviation of score differences between the first
and second tests for the entire group (percentage score).
c
GRI-R⫽Guyatt responsiveness index for responsiveness.
Usually, DQs and percentile scores can be used to
evaluate the severity of developmental delay,4,38 and raw
scores and percentage scores can be used for quantifying
the effect of intervention.10,27 In this study, the reliability
of the DQs, percentile scores, raw scores, and percentage
scores of the PDMS-2 was investigated, and high levels of
test-retest reliability were demonstrated for children with
CP. A previous study showed that the test-retest reliability
coefficients of the DQs for the PDMS-2 were .73 to .89
for children developing typically and aged 2 to 11
months and .93 to .96 for those aged 12 to 17 months.11
Because of differences in samples, the reliability coefficients are not directly comparable. Previous studies did
not provide information on test-retest reliability for
children with CP. As indicated by the results of this
study, various scales of the PDMS-2 are reliable for use in
clinics for motor skill acquisition or development for
children with CP.
We found that the SEMs of the PDMS-2 obtained in this
study were rather small, indicating that the error band of
the observed scores was limited. Compared with the
SEMs for the DQs of the norm samples of the PDMS-2
for children aged 24 to 72 months (3– 4 for GM, 2–5 for
FM, and 2–3 for TM),11 the SEMs for children with CP in
this study were lower. Because the SEM is inversely
related to the reliability coefficient, a relatively higher
reliability coefficient may cause a lower SEM. The value
of the SEM for a measure is useful for interpreting
whether a change or difference in scores is beyond
measurement error (ie, reaching real change or difference) in clinical settings. A higher criterion (SEM of
1.96) has been suggested for determining whether a
change for a child with CP is real (ie, beyond measurement error).39 For example, the raw score on the TM
composite for a child with CP should change more than
9.2 (ie, 1.96⫻4.7) for the change to be claimed as a real
change with a 95% confidence level. On the other hand,
if a child with CP has a change in the TM composite raw
score of less than 9.2, it cannot be interpreted as a real
improvement because such a change may be caused by
measurement error.
Note that a change beyond measurement error does not
necessarily indicate clinical relevance. Change beyond
measurement error is the minimum level representing
meaningful change. A clinically relevant change on a
scale can be determined by combining both distributionbased methods (eg, SEM) and anchor-based methods
(eg, parents’ or clinicians’ judgments).40 Although caregivers’ perceptions about overall change on the GM, FM,
or TM composites were determined with an anchorbased method in this study, the lack of clinicians’
judgment and the modest sample size in this study limit
the data for determining minimal clinically important
change in the PDMS-2. Future studies to determine the
benchmarks of minimal clinically important change in
the PDMS-2 are warranted for clinicians to interpret
their data.
This study also revealed that the percentage scores on
the 3 composites of the PDMS-2 could be used for
evaluating motor change in children with CP and receiving therapy. According to Liang,41 sensitivity to change is
a necessary but insufficient condition for responsiveness.
For a test to be relevant or meaningful to the decision
maker, the responsiveness of the test should be provided.41 Our study revealed not only acceptable sensitivity
to change but also acceptable responsiveness of the
PDMS-2 for children with CP. The GRI-R values of the
PDMS-2 for children with CP were 1.7 to 2.3 in our study.
The magnitude of these statistics is comparable to that of
values obtained for other outcome measures. For example, the GRI-R value of the motor component of the
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Wang et al . 1357
Functional Independence Measure for stroke was 1.29.42
Few previous responsiveness studies for children with CP
used many of the change indexes suggested by Husted
et al35 to determine the validity of an evaluative tool.10,29
To select proper outcome instruments, clinicians should
consider the child’s age and diagnosis, the purpose of
testing, the reliability and responsiveness of the instruments, and the interpretability of the outcomes of the
instruments.27 The previous study with the GM composite of the first edition of the PDMS (PDMS-GM) for
infants with CP showed that the PDMS-GM had limitations when used as an evaluative measure for infants with
CP.10 However, the change score on the PDMS-GM was
not significantly different from that on the GMFM-88 for
infants with CP over a 6-month period.27 Previous studies
did not examine the change indexes of the PDMS-2. Our
study provides sensitivity-to-change and responsiveness
coefficients for the GM composite of the PDMS-2 as well
as for the FM and TM composites. Our study also
provides evidence for clinicians and researchers to confidently use the percentage scores of the PDMS-2 to
detect a motor change for children with CP.
For sensitivity-to-change coefficients, the SRM may be
preferred over the paired t value and the ES because the
paired t value is influenced by sample size26,35 and the
SRM, which uses the between-subject variability of individual change scores over time, provides more appropriate standardization than does the ES.37 Although a high
between-subject variability of individual scores may have
caused low ES values for children with CP in our study,
the ES values for 3 composites of the PDMS-2 still met
the minimum criterion (0.2) of Cohen.34 Guyatt et al37
suggested that the GRI was the most appropriate measure of responsiveness because it used the variability of
change scores in stable subjects to standardize the clinically important difference; however, its assumption that
the variance in stable subjects is approximately equal to
the variance in an improved subject may induce biased
estimation.41 At present, no single change index is
superior. We provided 4 sensitivity-to-change coefficients
and 1 responsiveness coefficient for the percentage
scores of the PDMS-2 for children with CP in this study.
Advance knowledge of the responsiveness coefficient of
an instrument would permit the accurate estimation of
the sample size needed for adequate statistical power.37,43 If the GRI for the PDMS-2 is known, then the
sample size needed for any experiment in which change
over time in the PDMS-2 is the end point can be chosen
immediately. According to the table in the report by
Guyatt et al,37 for example, to detect a 3.2% mean
change in the GM composite score (GRI-R⫽1.7),
approximately 11 children per group would be required
for a study with unpaired observations or 7 per group
would be required for a study with paired observations.
1358 . Wang et al
To detect a 4.4% mean change in the FM composite
score (GRI-R⫽2.3), the required sample size would be
approximately 7 for unpaired observations or 5 for
paired observations. To detect a 3.4% mean change in
the TM composite score, the required sample size would
be similar to that for the FM composite score.
The sample size in this study was modest, although it is
reasonable for a clinical study. To improve the representation of the study sample, we tried to recruit children
with different types of CP in this study. Furthermore, our
evidence regarding test-retest reliability and responsiveness was acceptable. With the purposes that we proposed
and the results that we obtained, our results might not
be threatened by the modest sample size. In addition, we
used a retrospective global rating scale with moderate
reliability to calculate the responsiveness coefficient in
this study. Although we used the scores from 2 repetitive
rating scales to confirm the clinically important score
change in this study, a retrospective computation of
responsiveness has been criticized.28 The retrospective
global rating scale was valued lower than the prognostic
global rating scale by Stratford and Riddle.1 However,
the prognostic global rating scale can be used only by
clinicians and not by caregivers. The retrospective global
rating scale used to assess the importance and magnitude of a measured change is critical if health status
measures are to have an effect on patient care.41 Further
studies are needed to develop a valid external criterion
significant for both clients and clinicians. The further
study of psychometric properties (eg, minimal clinically
important change) is warranted to fully explore the utility
of the PDMS-2 for children with different types of CP.
Conclusion
The results of this study showed that the PDMS-2 had
good test-retest reliability over a 1-week period and
acceptable sensitivity to change and responsiveness over
a 3-month period for children with CP. The percentage
scores on 3 composites of the PDMS-2 could be used
over time to measure motor skill and motor development change over time for children with CP and aged
from 2 to 5 years. The criterion of an SEM of 1.96
(GM⫽2.6%, FM⫽2.6%, and TM⫽2.2%) could be used
to determine whether an individual achieves real change
(ie, beyond measurement error).
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Physical Therapy . Volume 86
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2006
Wang et al . 1359
Reliability, Sensitivity to Change, and
Responsiveness of the Peabody Developmental
Motor Scales−Second Edition for Children With
Cerebral Palsy
Hsiang-Hui Wang, Hua-Fang Liao and Ching-Lin Hsieh
PHYS THER. 2006; 86:1351-1359.
doi: 10.2522/ptj.20050259
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