J. Sleep Res. (2006) 15, 309–316
Development of the Sleep Quality Scale
H Y E R Y E O N Y I 1 , K Y U N G R I M S H I N 2 and C H O L S H I N 1 , 3
1
Institute of Human Genomic Study, College of Medicine, Korea University, Seoul, 2College of Nursing Science, Ewha Womans University, Seoul
and 3Division of Pulmonary and Critical Care Medicine, Department of Internal Medicine, Korea University Ansan Hospital, Ansan, Republic of
Korea
Accepted in revised form 26 June 2006; received 22 December 2005
SUMMARY
Sleep quality affects health and the overall quality of life. As the factors that influence
sleep quality and their relative importance vary among individuals, a self-report method
is essential. Although various questionnaires have been used to assess sleep quality, few
all-inclusive assessment scales have been developed. Therefore, the purpose of this study
was to develop an instrument for measuring sleep quality and to study its validity and
reliability. A Sleep Quality Scale (SQS) was developed using item analysis and factor
analysis on items with content validity. SQS, composed of 28 items and six factors,
accounted for 62.6% of the total variance. The difference of SQS score between
insomniacs and normal subjects confirmed the construct validity (t ¼ )13.8,
P ¼ 0.000). Concurrent validity was identified by the significant correlation of SQS
with the Pittsburgh Sleep Quality Index (r ¼ 0.72, P ¼ 0.000). The Cronbach’s alpha
coefficient was 0.92 for internal consistency and the correlation coefficient was 0.81 for
test–retest reliability at a 2-week interval. The developed SQS was therefore confirmed
to be a valid and reliable instrument for the comprehensive assessment of sleep quality.
keywords
reliability, sleep quality scale, validity
INTRODUCTION
Sleep is essential for restoration of the body and for
maintaining energy (Shapiro and Flanigan, 1993; Spensely,
1993). As sleep quality affects health (Edell-Gustafsson et al.,
2002; Ersser et al., 1999; Tynjala et al., 1999), daytime
function (Alapin et al., 2000; Chesson et al., 2000) and quality
of life (Carpenter and Andrykowski, 1998; Iliescu et al., 2003),
the measurements of sleep quality are important in both
clinical practice and research.
Both subjective and objective methods have been used to
assess sleep quality. Polysomnography, which is considered the
gold standard, provides accurate information on the physiological indices of sleep quality by using electroencephalogram,
electrooculogram and electromyelogram. However, it is expensive, needs technical training and also requires much time for
testing and the interpretation of data. In addition, it is difficult to
assess natural or usual sleep quality as a lot of equipment is
Correspondence: Chol Shin, MD, PhD, FCCP, Division of Pulmonary
and Critical Care Medicine, Department of Internal Medicine, Korea
University Ansan Hospital, 516, Gojan 1-dong, Danwon-gu, Ansan-si,
Gyeonggi-do 425-707, Republic of Korea. Tel.: +82-31-412-5603; fax:
+82-31-412-5604; e-mail: shinchol@pol.net
Ó 2006 European Sleep Research Society
required for its operation. Actigraphy is another method for
objective sleep monitoring that measures a person’s sleep
activity round the clock. Although it can measure sleep quality
and quantity for a week or more (Sadeh et al., 1994), it is limited
because it cannot distinguish between an awakened state
without movement and true sleep (Menefee et al., 2000). On
the other hand, self-report methods providing an assessment of
the sleep quality experienced by the sleeper include the sleep
diary, sleep log and sleep questionnaire. Although these methods do not provide information about the sleep structure, they
attempt to measure both quantitative and qualitative aspects of
sleep (Gentili et al., 1995). These subjective methods are easily
administered, inexpensive, and applicable to large surveys.
Sleep is a private experience that is influenced by many
factors including age, sex, food, and physical and psychological health. Components of sleep quality and their importance
differ according to individuals (Buysse et al., 1989), and thus a
self-report-based evaluation is essential to measure sleep
quality. Although various questionnaires have been used to
assess sleep quality, they feature several limitations. First,
there are few all-inclusive assessment scales. The previous
scales include items that are unrelated to sleep quality, or they
exclude essential items. There are also few questionnaires
which provide a total score.
309
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H. Yi et al.
Therefore, we undertook this study to develop an instrument to measure comprehensively the sleep quality of adults,
and subsequently to evaluate its validity and reliability.
METHOD
Instrument development
A methodological design was used. A scale was developed
according to the following process using classical test theory
(Fig. 1).
Definition of the concept
No scale can be constructed to measure a concept until the
nature of the concept has been delineated. Therefore, firstly
sleep quality was defined through a literature review.
and then their contents were confirmed by an in-depth
interview. These interviews were conducted with the consent
of subjects visiting a sleep laboratory and a research institute.
The subjects (32 males, 18 females) were 50 adults aged 21–
59 years. They comprised 17 subjects with insomnia symptoms, 17 with obstructive sleep apnea, six with snoring, two
with narcolepsy, and eight good sleepers. The main interview
questions were ÔHow is your sleep these days?Õ, ÔWhat do you
think a good sleep is?Õ, and ÔWhat do you think of a poor
sleep?Õ. The interview took about 30–60 min for each subject.
The subjects underwent one or two interviews which were
recorded and later transcribed. Content analysis of the
interview data was conducted line-by-line and the results were
confirmed by two nursing college professors with considerable
experience of qualitative research.
Review of items
Design of the scale
Two procedures were used to generate items to fully reflect the
concept of sleep quality. First of all, domains and their
contents of sleep quality were identified by a literature review
The constructed items were reviewed twice at a 2-week interval
by six experts with an academic background in sleep research
or clinical experience treating patients with sleep problems.
The experts evaluated whether items were valid using a fourpoint ordinal rating scale, where 1 denotes an irrelevant item
and 4 an extremely relevant item. The index of content validity
(CVI), the most widely used qualification of content validity,
indicates the proportion of items that received a rating 3 or 4
by the experts (Waltz and Bausell, 1981). Items with a CVI of
at least 0.8 were selected according to Lynn’s (1986) criteria.
The experts also evaluated the itemsÕ clarity, conciseness, and
redundancy. In addition, they also identified whether essential
items of sleep quality were excluded. The Korean language
vocabularies of the items were revised to a sixth grade reading
level and language usage was confirmed by consultation with a
Korean literature scholar and four elementary school teachers.
Pilot study
A pilot study was conducted twice to evaluate the degrees of
understanding concerning the sentences and vocabularies. As a
reading level between the fifth and seventh grades is appropriate for most instruments used in the general population
(Devellis, 2003), data for the first pilot study were gathered
from sixth grade elementary school students, while data for the
second pilot study were collected from the adults of the target
population. The time required to complete the questionnaire
was also surveyed.
Field test
Figure 1. Process of scale construction.
This step requires the collection of data for item analysis and
factor analysis. It has been reported that item analysis requires
a sample size of about 100–200 respondents (Spector, 1992)
while factor analysis requires a sample that is 10 times larger
than the number of items (Nunally, 1978). Data on the 46
items with content validity were collected by trained interviewers from community-dwelling adults aged 20–59 years.
Ó 2006 European Sleep Research Society, J. Sleep Res., 15, 309–316
Development of the Sleep Quality Scale
Selection of the final items
The final items of the scale to measure sleep quality were
selected by item analysis and factor analysis. Item analysis was
conducted to identify the items that form an internally
consistent scale. Item analysis shows the correlation between
respective items and the summated scale score. The higher the
coefficient for each item the more clear that the item belongs to
the scale. Generally, a coefficient of less than 0.3 suggests that
the item should be dropped from the scale (Anthoney, 1999; De
Vaus, 2002; Fisher et al., 2001). Thus the items with an itemtotal correlation of less than 0.3 were deleted from the scale.
Factor analysis was also used to reduce the number of items
and sort out the meaning. Among the several methods used in
the factor analysis, principal axis factoring and oblimin
rotation were used to extract the factors. Factors of scale
were selected by considering a scree test and the possibility of a
theoretical interpretation. Items with a factor loading of ±0.4
or less, or with a difference between absolute values of factor
loadings of 0.1 or less, were eliminated.
Evaluation of validity and reliability
The validity and reliability of a developed scale must be
evaluated by the collection of additional data from large
samples (Burns and Grove, 2001). Two convenience samples
were used to evaluate the validity and reliability. The first
sample, comprising 629 community-dwelling adults aged 18–
59 years, was used for construct validity, concurrent validity
using the Pittsburgh Sleep Quality Index (PSQI; Buysse et al.,
1989) and internal consistency reliability study. Data on the
Sleep Quality Scale (SQS), PSQI, questions defining insomnia,
sleep problems and demographic variables were collected from
adults by trained interviewers. After the exclusion of 17
incomplete responses, data from 612 subjects, 47.9% male and
52.1% female, of mean age 36.2 years, were analyzed.
Sample 2 comprised 110, 4-year university student subjects,
43.1% male and 56.9% female, with a mean age of 20.6 years.
These subjects completed SQS twice at an interval of 2 weeks
to confirm test–retest reliability.
Concurrent validity
Concurrent validity can be tested by identifying the correlation
between a new instrument and a previously validated instrument for measuring the same concept (Seong, 2002a).
Pittsburgh Sleep Quality Index was chosen as the criterion
because it is a scale that has been used widely to assess sleep
quality and disturbances over a 1-month time interval. It
consists of 19 items on seven component scores: subjective
sleep quality, sleep latency, sleep duration, sleep efficiency,
sleep disturbance, use of sleep medicine, and daytime dysfunction. The possible score ranges from 0 to 21, with a higher
score representing worsened sleep quality. The Cronbach’s
alpha coefficient of PSQI was reported at its inception as 0.83
(Buysse et al., 1989). The Cronbach’s alpha coefficient of PSQI
Ó 2006 European Sleep Research Society, J. Sleep Res., 15, 309–316
311
in this sample was 0.76. The correlation coefficient was
produced between SQS and PSQI using sample 1.
Construct validity
The validity of the instrument can be tested by identifying the
groups that are expected to have contrasting scores on the
instrument. Samples are then selected from at least two groups
that are expected to have opposing responses to the items in
the instrument (Burns and Grove, 2001).
As insomnia is a typical sleep disorder featuring the
complaint of insufficient or poor sleep quality, the sleep quality
of insomniacs was compared with that of normal subjects using
sample 1. The presence and frequency of four insomnia
symptoms – Ôdifficulty in initiating sleepÕ, Ôdifficulty in maintaining sleepÕ, Ôearly morning awakeningÕ, and Ônon-restorative
sleepÕ – were assessed according to previous studies (Ohayon
and Hong, 2002; Ohayon and Partinen, 2002). Insomnia was
considered to be present when it had occurred three or more
times a week over the previous month. In sample 1, there were
191 subjects with insomnia and 332 subjects without any of the
following sleep problems: insomnia, snoring, obstructive sleep
apnea, periodic limb movement disorder, and narcolepsy.
Reliability
The reliability of a measure denotes the consistency of
measures obtained in the use of particular instruments and is
an indication of the extent of random error in the measurement method (Burns and Grove, 2001). Two methods were
used to test the reliability of SQS.
Homogeneity testing examines the extent to which all the
items in the instrument consistently measure the construct. The
Cronbach’s alpha coefficients, the statistical procedure used in
this process, were computed using sample 1 to identify the
internal consistency of SQS. Stability, meanwhile, is concerned
with the consistency of repeated measures of the same attribute
with the use of the same scale or instrument. The correlation
coefficients were computed using sample 2 to identify the
stability of SQS.
RESULTS
Definition of sleep quality
Sleep can be defined as a regular, recurrent, easily reversible
state of the organism that is characterized by relative quiescence and by a large increase in the threshold of response to
external stimuli relative to the waking state (Kaplan and
Sadock, 1988). Quality is defined as the degree of excellence of
a thing or a general excellence (Oxford English Reference
Dictionary, 1995). That is, sleep quality means the degree of
excellence in sleep. It has been measured by many researchers
on the basis of this definition (Freedman et al., 1999; Hawkins
and Shaw, 1992; De Koninck et al., 1983; Mayers et al., 2003;
Shaver et al., 1991; Sickel et al., 1999). Based on a literature
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H. Yi et al.
review of previous questionnaires and studies, the domains of
sleep quality were identified as sleep initiation, sleep maintenance, depth of sleep, dreams, getting up after sleep, condition
after sleep, effect on daily life, sleep amount and satisfaction
with sleep.
comprising community-dwelling adults aged 20–59 years by
convenience sampling for item analysis and factor analysis.
The analyses were based on 817 subjects, 49.3% male and
50.7% female, of mean age 38.9 years, after 198 incomplete
data were excluded.
Design of the scale
Selection of the final items
Seventy-five items were generated in nine domains through the
content analysis of interview data and literature review. The
number of items in each domain was as follows: four items in
sleep initiation, five in sleep maintenance, four in depth of
sleep, three in dreams, six in getting up after sleep, 23 in
condition after sleep, 23 in effect on daily life, three in sleep
amount, and four in satisfaction with sleep. A four-point
rating scale was used to avoid a neutral response.
A new item of sleep efficiency was calculated by using three
items (time to go to bed, time to get up, actual sleeping time)
before item analysis. This produced a total of 44 items. From
the results of item analysis of 44 items, four items with an itemtotal correlation coefficient of under 0.3 were excluded to leave
40 items remaining.
Factor analysis on these 40 items yielded six factors,
including 28 items, which explained 62.6% of the total variance
in Table 1. Factor 1 included 12 items and was termed the
daytime dysfunction. It was composed of items representing
symptoms resulting from poor sleep, which accounted for
30.8% of the variance. Factor 2 included four items which
accounted for 10.8% of the variance. It was termed restoration
after sleep, which seemed to measure the primary function of
sleep. Factor 3 included four items which accounted for 8.1%
of the variance. It was termed difficulty in falling asleep and was
composed of items about sleep initiation. Factor 4 included
three items which accounted for 5.3% of the variance. It was
termed difficulty in getting up. Factor 5 included three items
which accounted for 3.9% of the variance. It was termed
satisfaction with sleep and was associated with the level of
overall gratification and sufficient sleeping time. Factor 6
included two items which accounted for 3.7% of the variance.
It was associated with awakening during sleep and was termed
difficulty in maintaining sleep. The correlation coefficients
between these factor scores ranged from 0.15 to 0.63 (Table 2).
Review of items
The index of content validity was calculated using the expertsÕ
ratings. Thirteen items had a CVI of under 0.8; 11 were
excluded and two were revised. Five items were also excluded
because of redundancy while a further two were added on the
basis of the expertsÕ opinion. This produced a total of 61 items.
After 14 days, the content validity on the 61 items was reevaluated by the same experts, resulting in four items with a
CVI of under 0.8 being excluded. A further 11 items with
duplicated meanings were excluded after discussion among the
experts, leaving 46 items in the preliminary scale.
Pilot study
The number of subjects in the first pilot study was five, sixth
grade school students. It took 10–15 min for them to complete
a questionnaire. The five students were asked whether or not
they understood the sentences and words in the questionnaire.
All five completely understood the meaning of 43 items and
four of them understood the meaning of two items. None of
the students understood the meaning of the last item: actual
sleeping time. Therefore, the following sentence was added to
clarify the meaning: Ôactual sleeping timeÕ is different from
Ôtime spent lying in bedÕ.
The second pilot study was conducted with the other
representative target population of 43 adults who visited an
outpatient department of a hospital and a research institute. It
also took 10–15 min for them to complete the questionnaire.
All of them understood the meaning of all items well. The
following instruction was supplemented to ensure that the
meaning of the response categories was clear: ÔfewÕ denotes
none or 1–3 times a month, ÔsometimesÕ 1–2 times a week,
ÔoftenÕ 3–5 times a week, and Ôalmost alwaysÕ 6–7 times a week.
Field test
As the aim of this scale was to measure adult sleep quality,
1015 data sets were collected from the target population
Evaluation of validity and reliability
Concurrent validity
Table 3 shows that SQS was strongly correlated with PSQI
(r ¼ 0.72, P ¼ 0.000). All the subscales were positively correlated with the PSQI total score.
Construct validity
Table 4 showed that the total score of the insomniacs was
significantly higher than that of the normal subjects
(t ¼ )13.89, P ¼ 0.000). All subscales had significant differences between the two groups.
Reliability
The SQS homogeneity was high with a Cronbach’s alpha
coefficient of 0.92. The Cronbach’s alpha coefficients of the
subscales ranged from 0.61 to 0.90: daytime dysfunction, 0.90;
restoration after sleep, 0.84; difficulty in falling asleep, 0.80;
Ó 2006 European Sleep Research Society, J. Sleep Res., 15, 309–316
313
Development of the Sleep Quality Scale
Table 1 Rotated factor matrix of Sleep Quality Scale items (n ¼ 817)
Items
F1
F2
F3
F4
F5
F6
Difficulty in thinking due to poor sleep
Difficulty in concentrating due to poor sleep
Increase of mistakes due to poor sleep
Irritated feeling due to poor sleep
Decrease of interest in work or others due to poor sleep
Getting tired easily at work due to poor sleep
Sleepiness that interferes with daily life
Painful life due to poor sleep
Decrease of desire due to poor sleep
Increase of forgetfulness due to poor sleep
Headache due to poor sleep
Decrease of appetite due to poor sleep
Relief of fatigue after sleep
Regaining vigor after sleep
Clear-headed feeling after sleep
Refreshed feeling of body after sleep
Difficulty in getting back to sleep after nocturnal awakening
Never falling asleep after awakening during sleep
Difficulty in falling asleep
Tossing and turning sleeplessly
Wish for more sleep after getting up
Difficulty in getting up after sleep
Feeling unlikely to sleep after sleep
Satisfaction with sleep
Deep sleep
Enough sleep time
Waking up easily due to noise
Waking up during sleep
Eigenvalue
Variance
Cumulated variance
0.78
0.75
0.75
0.71
0.69
0.69
0.69
0.67
0.64
0.59
0.59
0.45
0.01
)0.04
0.02
0.03
0.00
0.02
0.01
0.05
)0.01
0.06
0.15
)0.04
)0.03
0.04
)0.01
0.05
8.63
30.79
30.79
)0.04
)0.01
0.01
0.03
)0.03
)0.01
0.02
0.04
0.00
0.05
)0.03
0.01
0.82
0.81
0.77
0.63
)0.01
0.02
0.00
0.07
)0.01
0.04
0.14
0.02
0.01
0.14
0.00
0.00
3.01
10.76
41.55
0.00
0.02
)0.02
)0.10
0.00
0.01
)0.04
)0.06
0.06
0.02
0.14
0.10
0.04
0.01
)0.02
)0.02
0.78
0.75
0.58
0.50
)0.10
0.10
0.22
0.04
0.08
)0.07
0.02
0.02
2.28
8.14
49.70
)0.03
0.08
)0.06
0.08
)0.03
0.12
0.05
)0.09
0.08
)0.05
0.02
0.03
)0.10
)0.03
0.06
0.09
0.01
)0.10
0.12
0.12
0.84
0.70
0.48
0.06
)0.16
0.27
)0.03
0.01
1.47
5.26
54.96
0.06
0.00
)0.04
0.02
0.04
0.01
0.01
0.03
)0.04
)0.03
)0.02
)0.02
0.01
)0.06
0.00
0.11
)0.02
0.00
0.10
0.10
0.00
)0.04
0.12
0.78
0.72
0.42
0.02
0.01
1.08
3.88
58.84
)0.10
)0.03
)0.03
0.10
)0.03
0.05
)0.05
0.03
0.05
0.06
0.02
0.06
)0.05
0.05
0.03
)0.04
0.05
)0.03
0.08
0.13
0.05
)0.04
)0.03
)0.04
0.09
0.02
0.72
0.70
1.04
3.74
62.58
Entries in bold indicate where P-value is unnecessary.
Table 2 Correlations between factor scores in SQS (n ¼ 817)
Daytime
dysfunction
Restoration after sleep
Difficulty in falling asleep
Difficulty in getting up
Satisfaction with sleep
Difficulty in maintaining sleep
0.35
0.43
0.45
0.30
0.32
(P
(P
(P
(P
(P
¼
¼
¼
¼
¼
0.000)
0.000)
0.000)
0.000)
0.000)
Restoration
after sleep
0.30
0.40
0.61
0.25
(P
(P
(P
(P
¼
¼
¼
¼
difficulty in getting up, 0.74; satisfaction with sleep, 0.71; and
difficulty in maintaining sleep, 0.61.
The correlation coefficient for the 2-week interval test–retest
reliability was 0.81 (P ¼ 0.000) and the correlation coefficients
of the subscales were daytime dysfunction, 0.72 (P ¼ 0.000);
restoration after sleep, 0.74 (P ¼ 0.000); difficulty in falling
asleep, 0.56 (P ¼ 0.000); difficulty in getting up, 0.62
(P ¼ 0.000); satisfaction with sleep, 0.70 (P ¼ 0.000); and
difficulty in maintaining sleep, 0.68 (P ¼ 0.000).
DISCUSSION
The SQS was developed systematically and was confirmed to
be a valid and reliable instrument for the comprehensive
Ó 2006 European Sleep Research Society, J. Sleep Res., 15, 309–316
0.000)
0.000)
0.000)
0.000)
Difficulty in
falling asleep
Difficulty in
getting up
Satisfaction
with sleep
0.19 (P ¼ 0.000)
0.43 (P ¼ 0.000)
0.63 (P ¼ 0.000)
0.32 (P ¼ 0.000)
0.15 (P ¼ 0.000)
0.37 (P ¼ 0.000)
assessment of sleep quality. It measures sleep quality of the
previous month and provides a global score. The scale is
composed of 28 items and six factors, including daytime
dysfunction, restoration after sleep, difficulty in falling asleep,
difficulty in getting up, satisfaction with sleep, and difficulty in
maintaining sleep. It is a four-point Likert scale (few ¼ 0,
sometimes ¼ 1, often ¼ 2, almost always ¼ 3). Scores on
items in factors 2 and 5 have to be reversed before all the
scores are summed. The range of score is from 0 to 84, with a
higher score indicating a lower sleep quality.
Although the names of SQS factors and number of items
were a little different from those of previous questionnaires or
scales, the SQS items themselves were similar. Compared with
PSQI, the present scale included more items about restorative
0.000)
0.000)
0.000)
0.000)
0.000)
0.000)
0.190)
¼
¼
¼
¼
¼
¼
¼
(P
(P
(P
(P
(P
(P
(P
0.44
0.49
0.24
0.17
0.49
0.30
0.05
0.000)
0.000)
0.000)
0.000)
0.007)
0.000)
0.001)
¼
¼
¼
¼
¼
¼
¼
(P
(P
(P
(P
(P
(P
(P
0.22
0.18
0.16
0.24
0.11
0.14
0.13
0.000)
0.000)
0.000)
0.000)
0.000)
0.000)
0.000)
¼
¼
¼
¼
¼
¼
¼
(P
(P
(P
(P
(P
(P
(P
0.42
0.35
0.26
0.37
0.19
0.30
0.43
0.000)
0.000)
0.000)
0.000)
0.003)
0.000)
0.000)
¼
¼
¼
¼
¼
¼
¼
(P
(P
(P
(P
(P
(P
(P
0.33
0.23
0.22
0.40
0.12
0.29
0.31
0.000)
0.000)
0.000)
0.000)
0.000)
0.000)
0.000)
¼
¼
¼
¼
¼
¼
¼
(P
(P
(P
(P
(P
(P
(P
0.33
0.26
0.23
0.22
0.23
0.33
0.17
0.000)
0.000)
0.000)
0.000)
0.000)
0.000)
0.000)
¼
¼
¼
¼
¼
¼
¼
(P
(P
(P
(P
(P
(P
(P
0.51
0.38
0.38
0.57
0.27
0.39
0.28
0.000)
0.000)
0.000)
0.000)
0.000)
0.000)
0.000)
¼
¼
¼
¼
¼
¼
¼
(P
(P
(P
(P
(P
(P
(P
0.67
0.56
0.48
0.50
0.46
0.56
0.33
0.000)
0.000)
0.000)
0.000)
0.000)
0.000)
0.000)
¼
¼
¼
¼
¼
¼
¼
(P
(P
(P
(P
(P
(P
(P
0.72
0.61
0.48
0.59
0.48
0.58
0.39
Total Score
Daytime dysfunction
Restoration after sleep
Difficulty falling asleep
Difficulty getting up
Satisfaction with sleep
Difficulty maintaining sleep
Total score
PSQI
Subjective sleep
quality
Sleep latency
Sleep duration
Habitual sleep
efficiency
Sleep
disturbances
Use of sleeping
medication
Daytime
dysfunction
H. Yi et al.
Table 3 Correlations between SQS and PSQI (n ¼ 612)
314
functions after sleep, difficulty in getting up and various
daytime dysfunctions because of poor sleep. However, sleep
efficiency, an important concept of sleep quality (Åkerstedt
et al., 1994a; Bliwise, 1992), was excluded because of an itemtotal correlation of under 0.3. It is likely that sleep efficiency
was a less sensitive concept to measure sleep quality over a 1month period because it was calculated from items about
rough sleep time for the previous 1 month. That is, it is
considered as an appropriate concept to measure sleep
quality only of the previous night.
On the other hand, a factor should include at least three
items for meaningful interpretation according to Zwick and
Velicer (1986). The sixth factor of this scale was composed of
only two items relating to awakening during the night. This
factor was selected because sleep fragmentation produced
disruption of nocturnal sleep (Roehrs et al., 1994) and sleep
continuity was closely reflected in sleep quality (Åkerstedt
et al., 1994b). Thus it will be necessary to construct more
items related to awakening during sleep in future study.
When a scale is developed, its validity and reliability must
be evaluated. The construct validity of several questionnaires
has been studied by factor analysis. The Post-sleep Inventory
was composed of 29 items originally. Factor analysis yielded
seven factors, including only 26 items, with a total variance of
54.5% (Webb et al., 1976). The Sleep Questionnaire consisted
of 55 items when it was developed, but factor analysis
produced seven factors, comprising 41 items, which explained
71.7% of the total variance (Domino et al., 1984). The
Verran Snyder-Halpern Sleep Scale (VSH Sleep Scale) was
reduced from eight items to seven items by factor analysis
and these items accounted for 60% of the variance (SnyderHalpern and Verran, 1987). SQS was made up of items with
construct validity confirmed by factor analysis. Six factors
composed of 28 items of SQS explained 62.6% of the total
variance. This surpassed the criterion for total variance of
60% as set by social science (Chai, 2003), and therefore was
considered to be sufficient to account for sleep quality.
In contrast, PSQI was composed of items that were based
on empirical and clinical experiences, rather than statistical
analysis, so the present result could not be compared with the
total variance of PSQI. PSQI was validated instead using the
group differences among good sleepers, poor sleepers with
depression, disorders of initiating and maintaining sleep, and
disorders of excessive somnolence (Buysse et al., 1989). Other
studies have also validated their sleep questionnaire by
comparing with different subject populations (Domino et al.,
1984; Webb et al., 1976). The construct validity of SQS was
also examined using the known-group technique. From the
result, the total score (t ¼ )13.89, P ¼ 0.000) and scores of
the subscales discriminated between the subjects with insomnia and the normal subjects. Thus construct validity was
confirmed in a similar manner to that of the previous scales
or questionnaires.
In addition, the concurrent validity of SQS was studied by
using PSQI as the criterion. The correlation result (r ¼ 0.72,
P ¼ 0.000) confirmed the validity of SQS as a scale to
Ó 2006 European Sleep Research Society, J. Sleep Res., 15, 309–316
Development of the Sleep Quality Scale
Table 4. Comparison of SQS scores between
insomniacs and normal subjects (n ¼ 523)
Total score
Daytime dysfunction
Restoration after sleep
Difficulty in falling asleep
Difficulty in getting up
Satisfaction with sleep
Difficulty in maintaining sleep
measure sleep quality. This was a similar result to the
correlation coefficient of 0.71 between the Korean Sleep Scale
A and VSH Sleep Scale (Oh et al., 1998). Although there are
no absolute criteria to evaluate validity, the validity of SQS as
an instrument to measure sleep quality was confirmed to be
high according to the criteria (0.6–0.8) suggested by Seong
(2002b).
Reliability was tested in two ways. First of all, the
Cronbach’s alpha coefficient for homogeneity was 0.92,
indicating a high degree of internal consistency. Burns and
Grove (2001) suggested that a slightly lower coefficient (0.8–
0.9) indicated an instrument that would reflect more richly the
fine discriminations in the levels of the construct. This result
was higher than that of the Sleep Questionnaire (Cronbach’s
alpha coefficient 0.76) and that of PSQI (Cronbach’s alpha
coefficient 0.83). Test–retest reliability was also examined for
consistency of repeated measures. The correlation coefficient
of 0.81 confirmed the good stability of the instrument. The
correlation coefficient for PSQI after an interval of 28.2 days
(range 1–265) in all groups, including the normal subjects,
depressives, disorders of initiating and maintaining sleep, and
disorders of excessive somnolence, was 0.85 (Buysse et al.,
1989). The correlation coefficient for PSQI after an interval of
19 days (range 3–44) in nursing home residents was 0.82
(Gentili et al., 1995). The correlation coefficient for the Sleep
Questionnaire after an interval of 10 weeks in the normal
subjects was 0.79 (Domino et al., 1984). This result was similar
to the above results although the test intervals differed in the
previous studies. On the other hand, it was a little lower than
the correlation coefficient of 0.90 for PSQI in patients with
insomnia at a 2-day interval in the study of Backhaus et al.
(2002). This was presumably because of the short interval of
the PSQI test.
In summary, SQS was developed using item analysis and
factor analysis on items with content validity. The construct
validity and concurrent validity were confirmed. The internal
consistency reliability and test–retest reliability were acceptably high. SQS can also be used to assess sleep quality easily
and rapidly and can provide assistance in planning interventions for sleep problems. In addition, it can be utilized in sleep
research. Future studies should be conducted for further
validation and standardization of the scale in various clinical
settings, including obstructive sleep apnea and narcolepsy
patients, and with a diverse range of age groups, including the
Ó 2006 European Sleep Research Society, J. Sleep Res., 15, 309–316
315
Insomniacs
(n ¼ 191),
Mean ± SD
Normal subjects
(n ¼ 332),
Mean ± SD
t-value
P-value
31.1
8.2
7.8
2.9
4.7
5.7
1.8
15.8
3.2
4.3
1.1
2.6
3.4
1.2
)13.89
)9.69
)12.81
)8.26
)11.20
)10.55
)4.72
0.000
0.000
0.000
0.000
0.000
0.000
0.000
±
±
±
±
±
±
±
13.61
6.46
2.75
2.86
2.19
2.44
1.69
±
±
±
±
±
±
±
9.06
3.96
3.10
1.51
1.78
2.35
1.23
elderly. The ability of the scale to discriminate among the
levels of sleep quality needs to be confirmed through further
research.
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