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Psychometric study of an English version of Perceived Stress Scale in minority adolescents
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Psychometric study of an English version of Perceived Stress Scale in minority adolescents
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Content
PSYCHOMETRIC STUDY OF AN ENGLISH VERSION OF
PERCEIVED STRESS SCALE IN MINORITY ADOLESCENTS
by
Chen Li
A Thesis Presented to the
FACULTY OF THE USC GRADUATE SCHOOL
UNIVERSITY OF SOUTHERN CALIFORNIA
In Partial Fulfillment of the
Requirements for the Degree MASTER OF SCIENCE
(BIOSTATISTICS)
December 2015
Copyright 2015 Chen Li
!
ii!
TABLE OF CONTENTS
List of Tables iii
List of Figures iv
Abstract v
Introduction 1
Methods 3
Results 6
Discussion and Conclusion 13
References 16
!
iii!
LIST OF TABLES
Table 1 Items of the three versions of the Perceived Stress Scale 2
Table 2 Demographic and descriptive statistics (n=160) 6
Table 3 Internal consistency reliability of Perceived Stress Scale 7
Table 4 Bland-Altman test of thee versions of Perceived Stress Scale 10
Table 5 Correlation between Perceived Stress Scale (PSS) and Zung Self-
Rating Anxiety Scale (SAS)
11
Table 6 Perceived Stress Scale (PSS) and Zung Self-Rating Anxiety Scale
(SAS) scores analyzed by gender and ethnicity
12
!
iv!
LIST OF FUFURES
Figure 1 Bland-Altman agreement test among three versions of Perceived
Stress Scale.
9
!
v!
ABSTRACT
Background: The aim of the current study was to evaluate the reliability and validity of
the 10-item Perceived Stress Scale (PSS-10) and short form (PSS-4) versus the full-
version (PSS-14) of the instrument using data collected as part of two parallel cross-
sectional studies of obese Latino and African American adolescents. Methods: Data were
available for 160 subjects Internal consistency reliability and alternate forms reliability of
the Perceived Stress Scale (PSS) instrument were evaluated by the computation of
Cronbach’s alpha (α) coefficients and by correlation and agreement analyses among 4-,
10-, and 14-item Perceived Stress Scales. Convergent validity was investigated by
inspecting correlation with the Zung Self-Rating Anxiety Scale (SAS). Results:
Cronbach’s alpha coefficients for the PSS-14, PSS-10, and PSS-4 were 0.63, 0.72, and
0.56, respectively. Correlations across the three versions of the Perceived Stress Scale
were significant and indicate strong linear relationship (Spearman's rho (ρ) coefficients
calculated on the total and subscale level were: 0.94, 0.86, and 0.87 (total scale); 0.91,
0.84, and 0.90 (coping subscale); and 0.97, 0.84, and 0.84 (stress subscale), reported
correlations were between PSS-14 and PSS-10, PSS-14 and PSS-4, and between PSS-10
and PSS-4 respectively; all correlations significant with P<0.01). Normalized total and
subscale scores of the three versions of the Perceived Stress Scale corresponded with
acceptable agreement according to the Bland-Altman test. Convergent validity of all
versions of the Perceived Stress Scale was verified by the significant (P<0.01)
correlations between perceived stress and anxiety scores (Spearman's rho coefficients
between Zung Self-Rating Anxiety Scale (SAS) and PSS-14, PSS-10, and PSS-4 total
scores were 0.55, 0.56, and 0.47 respectively). Conclusions: The 10-item version of the
Perceived Stress Scale (PSS-10) exhibits optimum internal reliability and validity as
compared to the 4- and 14-versions of the Perceived Stress Scale in the present study.
The results support subsequent analyses involving perceived stress in this population.
!
!
1
INTRODUCTION
Stress refers to a state of bodily or mental pressure resulting from diverse reasons (e.g.,
emotions, diseases) that tend to affect physiological and/or psychological homeostasis
(Chrousos, 1998). Prior research has demonstrated associations between stress and various
adverse health conditions (e.g., anxiety, depression, and cardiovascular disease) (Cohen, Janicki-
Deverts and Miller, 2007; Lombard, 2010; Sims et al., 2008). To support stress research, there
arises the need of efficient, valid and reliable scales to quantitatively measure the level of
perceived stress. Among the tools and instruments that measure perceived stress, the Perceived
Stress Scale (PSS) (Cohen, Kamarck and Mermelstein, 1983) is a commonly used instrument in
stress research studies (e.g., estimation of the stressfulness of events, stress control). The original
scale comprises 14 survey questions (PSS-14) and quantifies global stress based on respondents’
feelings or thoughts on events or situations experienced over the last month (Cohen et al., 1983;
Cohen, Spacapan and Oskamp, 1988). Two shortened versions (4-item PSS-4 and 10-item PSS-
10) were developed as subsets of the full-scale 14 items based on factor analysis (Cohen and
Williamson, 1988) and need for greater efficiency and convenience to deploy in certain research
settings. The items of the three versions of the Perceived Stress Scale are exhibited in Table 1.
(Cohen et al., 1983; Cohen and Williamson, 1988).
Since the Perceived Stress Scale is administered by questionnaire and interview, investigators
need to implement effective validation strategies within the sample for confirmation of reliability
and validity for proper use and interpretation of the obtained data. The scale has not been
previously validated in the proposed sample (obese Latino and African American adolescents)
and proper appraisals of the psychometric properties of the Perceived Stress Scale within this
population are essential for the appropriate application of the instrument.
The current study is aimed to compare three versions of the Perceived Stress Scale with 4, 10,
and 14 items (PSS-4, PSS-10, and PSS-14, respectively) using data from a parallel clinical study
(Diabetes Risk due to Ectopic Adiposity in Minority Youth (DREAM)) (Toledo-Corral et al.,
2013). The evaluation of psychometric properties of these versions of the PSS are aimed to
support use of the PSS in subsequent studies designed to evaluate a stress relief intervention and
estimate the association between eating habits and stress.
!
!
2
Table 1. Items of the three versions of the Perceived Stress Scale
Item PSS-14 PSS-10 PSS-4
1 Upset because of something that happened unexpectedly √
√
2 Felt that you were unable to control the important things in your life √ √ √
3 Felt nervous and “stressed” √ √
4 Dealt successfully with day to day problems and annoyances √
5 Felt effectively coping with important changes that were occurring in your life √
6 Felt confident about your ability to handle your personal problems √ √ √
7 Felt that things were going your way √ √ √
8 Found that you could not cope with all the things that you had to do √ √
9 Able to control irritations in your life √ √
10 Felt that you were on top of things √ √
11 Angered because of things that were outside your control √ √
12 Thinking about things that you have to accomplish √
13 Able to control the way you spend your time √
14 Felt difficulties were piling up so high that you could not overcome them √ √ √
!
!
!
3
METHODS
Participants and Procedures-
Participants- Perceived Stress Scale data were contemporaneously collected in a parallel clinical
study of the Diabetes Risk due to Ectopic Adiposity in Minority Youth (DREAM) study, a cross-
sectional investigation of relationships between ectopic fat distribution and T2DM risk in
overweight minority adolescents (Toledo-Corral et al., 2013). A total of 201 African American
and Latino-American adolescent males and females were recruited from Los Angeles County
according to the inclusion criteria of: 1) either African American (all four grandparents of Afro-
American descent) or Latino ethnicity (all four grandparents of Latin-American descent); 2) age
14–17 years; and 3) age- and gender-adjusted BMI≥ 85th percentile by the standards of the
Centers for Disease Control and Prevention (Ogden et al., 2004). Participants were excluded if
they had a previous major illness (e.g., type 1 or 2 diabetes, took medications) or had a condition
that influences body composition, insulin action, or insulin secretion. Participants and their
custodians provided written informed consent. The study was approved by the Institutional
Review Board of the University of Southern California (USC), Health Sciences Campus, Los
Angeles.
Procedures- As part of the medical visits for the larger study, all participants completed the 14-
item English version of the Perceived Stress Scale (PSS-14) and the Zung Self-Rating Anxiety
Scale (SAS) (Zung, 1971, 1974) on the same day; data were collected concurrently. The detailed
study procedures are described in a previous publication (Toledo-Corral et al., 2013) .
Measures-
Perceived Stress Scale (PSS)- The English version of the PSS-14 (Cohen et al., 1983; Cohen and
Williamson, 1988) was initially applied in the present study. Participants were asked to respond
to each item of the instrument on a 5-point Likert scale ranging from 0 (never) to 4 (very often),
reflecting the frequency of stressful feelings within the past month. Seven of the 14 items of the
PSS-14 are encrypted negatively (1, 2, 3, 8, 11, 12, and 14) and the remaining items are positive
(4, 5, 6, 7, 9, 10, and 13), representing “perceived stress” and “perceived coping” subscale,
respectively. The shorter 10-item and 4-item versions include the same items, though fewer of
them (Cohen and Williamson, 1988). The PSS-10 and PSS-4 contain four (6, 7, 9, and 10) and
!
!
4
two (6 and 7) positively stated items respectively (the remaining items are negative) (Table 1)
(Cohen et al., 1983; Cohen and Williamson, 1988). Total scores are calculated reversing the
scores of positive items and subsequent summing of all item scores; a higher score corresponds
to a higher level of perceived stress. The total score for the PSS-14 ranges from 0 to 56 (scores
range from 0 to 40 and from 0 to 16 for the PSS-10 and PSS-4 respectively).
Zung Self-Rating Anxiety Scale (SAS)- The Zung Self-Rating Anxiety Scale (SAS) is a 20-item
self-report assessment instrument designed to measure anxiety (Zung, 1971, 1974). Items of the
scale are based on manifestations of cognitive, autonomic, motor and central nervous system
symptoms. The questionnaire is composed of 20 statements and patients respond one of the 4
numbers (1 corresponds with none or a little of the time, 2 for some of the time, 3 for good part
of the time, and 4 for most or all of the time). Among the 20 questions, 5 (item 5, 9, 13, 17, and
19) are reverse coded; the total score is calculated by reversing the scores of corresponding items
and subsequently summing scores for all 20 statements.
Statistical Analyses-
Dataset management- Datasets of Perceived Stress Scale (PSS), Zung Self-Rating Anxiety Scale
(SAS), as well as 21- and 27-item Intuitive Eating Scale (IES-1 and IES-2) were merged with
demographic variables (i.e., gender and ethnicity) according to the subject number that is unique
for each participant. The histogram plot of Zung Self-Rating Anxiety Scale indicates a
substantial violation of normality assumption of the variable and data points that form the “tail”
zone were deleted for subsequent analysis. Data records with missing data in the combined
dataset were also deleted and the consequent dataset has a sample size of 160. Intuitive Eating
Scale data were prepared for ensuing analysis but not involved in the current study.
Intra-item reliability of the Perceived Stress Scale was evaluated by computing Cronbach’s alpha
(α) (Cronbach, 1951, 1970) for each subscale (i.e., stress and coping) and the full scale. A
coefficient value of 0.70, 0.80, and >0.90 indicate acceptable, adequate, and excellent internal
consistency, respectively (Jensen, 2003; McDowell and Newell, 1996). Alternate forms
reliability was evaluated by the calculation of Spearman’s correlation coefficients (Spearman,
1904) and Bland-Altman agreement test (extent of agreement was examined according to: 1)
discrepancy between scales; 2) width of the limit of agreement; 3) the association between the
!
!
5
difference and average of scores of the two scales; and 4) consistent variability of the difference
between the two scales across the average scores) (Bland and Altman, 1983; Bland and Altman,
1995; Bland and Altman, 1999; Nawarathna and Choudhary, 2013) conducted among the
normalized (adjusted to the scale of 0-16) total and subscale scores of the three versions of
Perceived Stress Scale (PSS-4, PSS-10, and PSS-14). Convergent validity was assessed by
correlating scores of each version of the Perceived Stress Scale (PSS) with the Zung Self-Rating
Anxiety Scale (SAS).
Stress scores were normalized to the same scale (0-16) for comparisons between different
versions of the Perceived Stress Scale (i.e., Bland-Altman agreement test and non-parametric
mean comparisons). Normalization is calculated by multiplying the percentage of stress with 16
(e.g., a total score of 28 on the PSS-14 scale has a percentage of 28/56×100%=50% and the
corresponding normalized score is 8 on the scale of 0-16). For both the Perceived Stress Scale
and the Zung Self-Rating Anxiety Scale datasets,!records with missing data were removed from
subsequent analyses for best accuracy (the combined dataset consequently has a sample size of
n=160).
Non-parametric statistical methods (i.e., Spearman’s correlation (Spearman, 1904) and Kruskal-
Wallis test (Wallis, 1952)) were used for correlation analysis and group comparisons on central
tendency; non-parametric methods were selected partially due to the potential violation of
parametric assumptions (i.e., linearity, normality, and homogeneity of variance (Geisser and
Johnson, 2006)). Statistical analyses were conducted using SPSS 22.0, SAS 9.4, and Prism 6.0.
All tests of significance were two-sided, and a P value<0.05 was considered significant.
!
!
6
RESULTS
Descriptive statistics- Data records with missing values as well as with data points that
dramatically violate the assumption of normality (i.e., anxiety scores forming an obvious “tail”
shape in the variable histogram) were deleted for best accuracy and the final sample size for
analysis is 160. Respondent demographics and Perceived Stress Scale scores are displayed in
Table 2. Among the 160 respondents, 45.6% (n=73) were females; 34.4% (n=55) were African-
American. Mean (SD) total perceived stress scores were 24.0 (6.7), 15.2 (5.9), and 5.7 (3.1) for
PSS-14, PSS-10, and PSS-4, respectively. Average (SD) total Perceived Stress Scale scores
normalized to the same scale of 0-16 were 6.8 (2.0), 6.1 (2.5), and 5.7 (3.1) for PSS-14, PSS-10,
and PSS-4, respectively. Mean Zung Self-Rating Anxiety Scale (SAS) score was 31.3 (6.1)
(Table 2).
Table 2. Demographic and descriptive statistics (n=160)!
Gender Male 87 (54.4%)
Female 73 (45.6%) !
Ethnicity African-American 55 (34.4%)
Latino 105 (65.6%) !
Perceived Stress Scale (PSS)- Scores analyzed on the original scale
PSS-14 PSS-10 PSS-4
Stress 10.4 ± 5.2 (0-24) 7.9 ± 4.6 (0-34) 2.3 ± 2.0 (0-8)
Coping 13.5 ± 5.6 (0-28) 7.3 ± 3.7 (0-16) 3.4 ± 2.1 (0-8)
Total Scale 24.0 ± 6.7 (5-42) 15.2 ± 5.9 (0-34) 5.7 ± 3.1 (0-14)
Perceived Stress Scale (PSS)- Scores normalized to the scale of 0-16
PSS-14 PSS-10 PSS-4
Stress 5.9 ± 3.1 (0-14) 5.2 ± 3.2 (0-14) 4.6 ± 4.0 (0-16)
Coping 7.7 ± 3.3 (0-16) 7.4 ± 3.8 (0-16) 6.9 ± 4.1 (0-16)
Total Scale 6.8 ± 2.0 (1-12) 6.1 ± 2.5 (0-14) 5.7 ± 3.1 (0-14)
Zung Self-Rating Anxiety Scale (SAS) 31.3 ± 6.1 (21-53) !!
Psychometric scores are shown as mean ± SD (range)!
Reliability analysis of Perceived Stress Scale-
Internal consistency of items- Cronbach’s alpha coefficients for the subscale and total scale of
PSS-14, PSS-10, and PSS-4 were 0.80, 0.80, and 0.75 (coping subscale); 0.78, 0.79, and 0.69
(stress subscale); as well as 0.63, 0.72, and 0.56 (total scale) respectively (Table 3). Results
indicate that PSS-14 and PSS-10 show marginally acceptable internal consistency but PSS-4
does not show acceptable internal consistency (Table 3). By comparison, PSS-10 shows
optimum internal consistency among the three versions of the Perceived Stress Scale and the
!
!
7
lower correlations with the PSS-4 draws concerns for using this very short version.
Table 3. Internal consistency reliability of Perceived Stress Scale!
Scale Cronbach's alpha
! PSS-14 PSS-10 PSS-4
Stress 0.78 0.79 0.69
Coping 0.80 0.80 0.75
Total 0.63 0.72 0.56
Alternate forms reliability- Pairwise correlation analyses revealed that total scores of PSS-4,
PSS-10, and PSS-14 were significantly correlated with one another (Spearman’s correlation
coefficients between total scores of PSS-14 and PSS-10, PSS-14 and PSS-4, and between PSS-
10 and PSS-4 among all participants were 0.94, 0.86, and 0.87, respectively with all correlations
significant at the 0.01 level). Corresponding coefficients correlate with strong (>0.80) linear
relationships and support the alternate forms reliability. Significant correlations were also
observed for the subscales (i.e., coping and stress) among the three versions of the Perceived
Stress Scale. Subscale Spearman’s rho coefficients were higher than the total scale correlations,
demonstrating consistency of subscales among the different versions of the Perceived Stress
Scale (Spearman’s correlation coefficients between normalized subscale scores of PSS-14 and
PSS-10, PSS-14 and PSS-4, and between PSS-10 and PSS-4 among all participants are 0.91,
0.84, and 0.90 (coping subscale) and 0.97, 0.84, and 0.84 (stress subscale) respectively with all
correlations significant at the 0.01 level).
The Bland-Altman test for the total PSS scale indicates acceptable agreement among the three
versions of the Perceived Stress Scale (biases between PSS-14 and PSS-10, PSS-14 and PSS-4,
and between PSS-10 and PSS-4 were 0.74, -2.26, and -1.11, respectively) (Table 4). Most
observations were located within the 95% agreement limit (Figures 1). Comparatively smaller
biases were observed at the subscale levels (coping subscale: biases between PSS-14 and PSS-
10, PSS-14 and PSS-4, and between PSS-10 and PSS-4 are 0.34, -0.86, and -0.52; stress
subscale: biases between PSS-14 and PSS-10, PSS-14 and PSS-4, and between PSS-10 and PSS-
4 are 0.70, -1.36, and -0.65); these correspond with good agreement of the coping and stress
subscales of the three versions of the Perceived Stress Scale. The widths of agreement limits are
approximately similar at both the subscale and total scale levels; the largest width was observed
between PSS-10 and PSS-4 at the total scale level (Table 4). At both subscale levels, there
!
!
8
observed no dramatic linear trend in the biases (divergence between scores of corresponding
scales) as values of the average (of scores of corresponding scales) increase whereas slight linear
associations between average and difference were identified at the total scale level. Results
indicate the three versions of the Perceived Stress Scale manifest greater agreements at the
subscale than at the total scale levels. Among all agreement tests, the variability of the biases is
roughly consistent across the average scores (Figure 1).
!
!
9
Figure 1. Bland-Altman agreement test among three versions of Perceived Stress Scale-
Comparisons are shown at the level of total scale (left panel), stress (middle panel) as well as
coping (right panel) subscales. Plots at the upper, middle, and lower panels correspond with
comparisons between PSS-14 and PSS-10, PSS-14 and PSS-4, and between PSS-10 and PSS-4,
respectively.
0 5 10 15
-4
-2
0
2
4
Average
Difference
Total Scale
PSS-14 vs PSS-10
0 5 10 15
-6
-4
-2
0
2
4
Average
Difference
Total Scale
PSS-14 vs PSS-4
0 5 10 15
-10
-5
0
5
10
Average
Difference
Total Scale
PSS-10 vs PSS-4
0 5 10 15
-2
-1
0
1
2
3
Average
Difference
Stress Subsclae
PSS-14 vs PSS-10
0 5 10 15 20
-10
-5
0
5
10
Average
Difference
Stress Subscale
PSS-14 vs PSS-4
0 5 10 15 20
-10
-5
0
5
10
Average
Difference
Stress Subscale
PSS-10 vs PSS-4
0 5 10 15 20
-6
-4
-2
0
2
4
6
Difference
Coping Subscale
PSS-14 vs PSS-10
Average
0 5 10 15 20
-5
0
5
10
Average
Difference
Coping Subscale
PSS-14 vs PSS-4
0 5 10 15 20
-10
-5
0
5
10
Average
Difference
Coping Subscale
PSS-10 vs PSS-4
!
!
10
Table 4. Bland-Altman test of thee versions of Perceived Stress Scale!
Bias SD of Bias 95% Limit of Agreement Width of Agreement Limit
Coping Subscale
PSS-14 vs PSS-4 0.86 2.27 -3.59 5.31 8.90
PSS-14 vs PSS-10 0.34 1.52 -2.64 3.32 5.96
PSS-10 vs PSS-4 0.52 1.79 -2.99 4.04 7.03
Stress Subscale
PSS-14 vs PSS-4 1.36 2.24 -3.02 5.74 8.76
PSS-14 vs PSS-10 0.70 0.72 -0.71 2.11 2.82
PSS-10 vs PSS-4 0.65 2.17 -3.61 4.91 8.52
Total Scale
PSS-14 vs PSS-4 -1.10 1.71 -4.45 2.23 6.68
PSS-14 vs PSS-10 0.74 0.89 -1.01 2.45 3.46
PSS-10 vs PSS-4 -2.26 3.29 -8.72 4.2 12.92
!
!
11
Convergent validity- Convergent validity was assessed by the correlation of the Perceived Stress
Scale and Zung Self-Rating Anxiety Scale (SAS) scores. According to Spearman’s correlation
analysis, PSS-14, PSS-10, and PSS-4 total scores were significantly correlated with Zung Self-
Rating Anxiety Scale scores. Spearman’s correlation coefficients between total scores of SAS
and PSS-14, SAS and PSS-10, SAS and PSS-4 among respondents are 0.55, 0.56, and 0.47,
respectively. Though moderate in association, all correlations were statistically significant at the
0.01 level (Table 5).
Table 5. Correlation between Perceived Stress Scale (PSS) and Zung Self-Rating Anxiety
Scale (SAS) !
Scale Spearman's rho
! PSS-14 PSS-10 PSS-4
Stress 0.45 0.30 0.40
Coping 0.25 0.47 0.45
Total 0.55 0.56 0.47
All correlations are significant at the 0.01 level (2-tailed)
Insensitivity of Perceived Stress Scale and Zung Self-Rating Anxiety Scale (SAS) scores as to
demographic variables- Kruskal-Wallis tests indicate non-significant inconsistency between
corresponding instrument scores according to gender and ethnicity subgroups and support the
notion that analyses are not significantly affected by possible influences attributed to these
variables. Descriptive statistics categorized by gender and ethnicities were shown in Table 6.
!
!
12
Table 6. Perceived Stress Scale (PSS) and Zung Self-Rating Anxiety Scale (SAS)
scores analyzed by gender and ethnicity
Variable
Scale PSS-14 PSS-10 PSS-4
Perceived Stress Scale (PSS)- Scores normalized to the scale of 0-16
Male
Stress 5.3 ± 2.9 (0-14) 4.6 ± 3.0 (0-12) 4.2 ± 3.9 (0-16)
Coping 8.1 ± 3.4 (1-16) 7.8 ± 3.9 (0-16) 7.0 ± 4.4 (0-16)
Total Scale 6.7 ± 2.0 (2-12) 5.9 ± 2.3 (0-12) 5.6 ± 3.0 (0-14)
Female
Stress 6.6 ± 3.1 (0-14) 5.9 ± 3.3 (0-14) 5.0 ± 4.2 (0-16)
Coping 7.3 ± 3.0 (0-16) 6.9 ± 3.6 (0-16) 6.7 ± 4.0 (0-16)
Total Scale 7.0 ± 2.0 (1-12) 6.3 ± 2.6 (0-14) 5.9 ± 3.1 (0-14)
African American
Stress 6.3 ± 3.3 (0-14) 5.6 ± 3.4 (0-14) 4.7 ± 4.0 (0-16)
Coping 7.1 ± 3.3 (0-16) 6.8 ± 4.1 (0-16) 6.2 ± 4.4 (0-16)
Total Scale 6.7 ± 2.3 (1-12) 6.7 ± 2.3 (0-14) 5.4 ± 3.5 (0-14)
Latino
Stress 5.7 ± 3.0 (0-14) 5.0 ± 3.1 (0-12) 4.5 ± 4.1 (0-16)
Coping 8.1 ± 3.2 (2-16) 7.7 ± 3.6 (2-16) 7.2 ± 4.1 (0-16)
Total Scale 6.9 ± 1.8 (2-12) 6.1 ± 2.3 (1-12) 5.9 ± 2.8 (0-12)
Zung Self-Rating Anxiety Scale (SAS)
Male 30.9 ± 5.6 (21-49)
Female 31.6 ± 6.7 (21-53)
African American 30.9 ± 6.6 (21-53)
Latino 31.4 ± 5.9 (22-49)
Psychometric scores are shown as mean ± SD (range)
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13
DISCUSSION AND CONCLUSION
The present study evaluated the reliability and validity of a 10-item English Version of the
Perceived Stress Scale (PSS-10) as compared to alternate versions of the scale (PSS-4 and PSS-
14) using the data from parallel clinical studies. The 10-item version of the scale (PSS-10)
manifested preferable psychometric properties (related to internal reliability and convergent
validity) as compared to PSS-4 and PSS-14, with marginally acceptable internal reliability and
excellent convergent validity with the Zung Self-Rating Anxiety Scale (SAS). PSS-14 and PSS-4
scores (obtained from the same sample) were utilized for alternate form reliability analysis and
results indicate significant correlations and consistent agreement among the three versions of the
Perceived Stress Scale (PSS-4, PSS-10, and PSS-14).
The optimal reliability of the PSS-10 among the three versions of the Perceived Stress Scale may
be partially attributed to the efficient design of the scale. PSS-10 is a relatively short version but
includes adequate items for evaluating stress. In a relatively small sample (n=160), PSS-10 tends
to yield a comparatively high internal reliability. While the longer PSS-14 should have higher
internal consistency, more items may introduce uncertainty and unreliability in the response data.
Concerning the small sample size in the current study, the 14-item Perceived Stress Scale may
not be the most advantageous instrument for evaluating stress. The shortest version PSS-4
demonstrated even lower internal reliability (Cronbach’s alpha 0.56). Present results suggest the
PSS-10 as the optimum scale in this specific sample for measuring perceived stress.
Correlation analysis was used in the present study to analyze alternate form reliability and
convergent validity. In consideration of the relatively small sample size, correlations were
conducted by the non-parametric method (Spearman’s correlation); consistent results were seen
using the Pearson’s correlation (data not shown). Correlation analysis is however limited in
determining agreement of scales since two sets of measurements can be perfectly correlated but
do not agree (Bland and Altman, 1983; Bland and Altman, 1995; Bland and Altman, 1999). The
Bland-Altman analysis evaluates the agreement of two measurement scales or methods by
visualizing discrepancy of measures of two scales against the mean of measurements (Bland and
Altman, 1995; Bland and Altman, 1999). In comparison to the correlation analysis, the method
provides a more reliable evaluation of actual agreement between two scales (Bland and Altman,
14
1983; Nawarathna and Choudhary, 2013). Consistent with the reliability results, Bland-Altman
analyses of these Perceived Stress Scale data indicated superior agreement (illustrated by
relatively smaller discrepancy (bias) between scales and slighter linear association of the score
difference with the average as contrast to the commensurate comparisons among total scales) at
subscale levels (coping or stress subscales) among the three version of the Perceived Stress
Scale. The total scale level analysis showed potential linear trends in the divergence between
scales across the plots (Figure. 1). The biases between corresponding scales and subscales ranged
from 0.34 to 2.26 (median=0.65); these biases may be considered not dramatic on the normalized
scale of 0-16.
Alternate form reliability among different versions of the Perceived Stress Scale was analyzed by
comparison of normalized average scores at the total as well as subscale levels (data not shown).
This method is not preferred as compared to the Bland-Altman analysis in part due to the
fluctuation of the data; when the standard deviation of one scale is large, it may be likely to
identify a non-significant disparity in means between that scale with another and a corresponding
false conclusion of the consistency of the two scales. In contrast, the comparison of means may
on occasion be sensitive to slight variances (as occurred in the present study) and a minor
inequality between scales may be statistically identified as significant, with a false conclusion
that the two scales are not consistent. Due to the inaccuracy in evaluating the equilibrium
between psychometric scales, the comparison of means is conducted only as a supportive
approach and the significance and non-significance of incongruence between scales should be
interpreted in collaboration with other statistical strategies, as was done in the present analysis.
Parametric statistical analyses (e.g., t-test and Pearson’s correlation) assume normality, linearity,
and variance homogeneity and violation of these suppositions may yield inaccurate conclusions.
Upon removal of records with missing data, the sample size was reduced to 160 and a number of
variables did not display normal distributions. Non-parametric Spearman’s correlation and
Kruskal-Wallis tests were selected for measuring the correlation and mean comparisons in
regards to alternate form reliability validation and descriptive data analysis. Another limitation of
this study lies with the validity analysis; the Perceived Stress Scale is currently only validated
with the Zung Self-Rating Anxiety Scale (SAS) for construct (convergent) validity.
Supplementary construct validity could involve Exploratory and Confirmatory Factor Analysis
15
(EFA/CFA) with a larger sample size to identify and corroborate the corresponding subscale
structure. In the current study, Confirmatory Factor Analysis of first-order and second-order two-
factor models was unable to be computed on PSS-4 and PSS-14 instruments; this could be
partially attributed to the limited sample size and relatively low internal consistency of the PSS-4
and PSS-14.
Conclusion- The 10-item version of the Perceived Stress Scale (PSS-10) presents marginally
acceptable reliability and excellent validity in the present study of a sample size of 160
overweight and obese African-American and Latino adolescents and is the optimal scale as
compared with 4-item and 14-item scales (PSS-4 and PSS-14) in terms of internal reliability,
alternate forms stability, and convergent validity. Results from this study support ensuing
analyses using these data in this specific sample.
!
!
16
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Cronbach, L. J. (1970). Essentials of Psychological Testing. Harper & Row., 161.
Geisser, S., and Johnson, W. M. (2006). Modes of Parametric Statistical Inference. Modes of
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Jensen, M. P. (2003). Questionnaire validation: a brief guide for readers of the research
literature. Clin J Pain 19, 345-352.
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studies with heteroscedastic measurements. Stat Med 32, 5156-5171.
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Ogden, C. L., Fryar, C. D., Carroll, M. D., and Flegal, K. M. (2004). Mean body weight, height,
and body mass index, United States 1960-2002. Adv Data, 1-17.
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community-based sample of African Americans. Eat Behav 9, 137-142.
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Journal of Psychology 15.
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prediabetic overweight and obese minority adolescents. J Clin Endocrinol Metab 98, 1115-1121.
Wallis, K. (1952). Use of ranks in one-criterion variance analysis. Journal of the American
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Zung, W. W. (1971). A rating instrument for anxiety disorders. Psychosomatics 12, 371-379.
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!
Abstract (if available)
Abstract
Background: The aim of the current study was to evaluate the reliability and validity of the 10-item Perceived Stress Scale (PSS-10) and short form (PSS-4) versus the full-version (PSS-14) of the instrument using data collected as part of two parallel cross-sectional studies of obese Latino and African American adolescents. Methods: Data were available for 160 subjects Internal consistency reliability and alternate forms reliability of the Perceived Stress Scale (PSS) instrument were evaluated by the computation of Cronbach’s alpha (α) coefficients and by correlation and agreement analyses among 4-, 10-, and 14-item Perceived Stress Scales. Convergent validity was investigated by inspecting correlation with the Zung Self-Rating Anxiety Scale (SAS). Results: Cronbach’s alpha coefficients for the PSS-14, PSS-10, and PSS-4 were 0.63, 0.72, and 0.56, respectively. Correlations across the three versions of the Perceived Stress Scale were significant and indicate strong linear relationship (Spearman's rho (ρ) coefficients calculated on the total and subscale level were: 0.94, 0.86, and 0.87 (total scale)
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Li, Chen
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Psychometric study of an English version of Perceived Stress Scale in minority adolescents
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Keck School of Medicine
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Applied Biostatistics and Epidemiology
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09/29/2015
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