ARTHRITIS & RHEUMATISM Vol. 46, No. 3, March 2002, pp 714–725 DOI 10.1002/art.10095 © 2002, American College of Rheumatology
The PedsQL™ in Pediatric Rheumatology Reliability, Validity, and Responsiveness of the Pediatric Quality of Life Inventory™ Generic Core Scales and Rheumatology Module James W. Varni,1 Michael Seid,2 Tara Smith Knight,2 Tasha Burwinkle,2 Joy Brown,2 and Ilona S. Szer1 physical health summary score (␣ ⴝ 0.87 for child self report, ␣ ⴝ 0.89 for parent proxy report), and psychosocial health summary score (␣ ⴝ 0.86 for child self report, ␣ ⴝ 0.90 for parent proxy report) were acceptable for group comparisons. The Rheumatology Module scales also demonstrated acceptable reliability for group comparisons (␣ ⴝ 0.75–0.86 for child self report, ␣ ⴝ 0.82–0.91 for parent proxy report). Validity was demonstrated using the known-groups method. The PedsQL distinguished between healthy children and children with rheumatic diseases as a group. The responsiveness of the PedsQL was demonstrated through patient change over time as a result of clinical intervention. Conclusion. The results demonstrate the reliability, validity, and responsiveness of the PedsQL 4.0 Generic Core Scales and the PedsQL 3.0 Rheumatology Module in pediatric rheumatology.
Objective. The Pediatric Quality of Life Inventory (PedsQL) is a modular instrument designed to measure health-related quality of life (HRQOL) in children and adolescents ages 2–18 years. The 23-item PedsQL 4.0 Generic Core Scales are multidimensional child selfreport and parent proxy-report scales developed as the generic core measure to be integrated with the PedsQL disease-specific modules. The 22-item PedsQL 3.0 Rheumatology Module was designed to measure pediatric rheumatology–specific HRQOL. This study was undertaken to demonstrate the reliability, validity, and responsiveness of the PedsQL 4.0 Generic Core Scales and the PedsQL 3.0 Rheumatology Module in pediatric rheumatology. Methods. The 4 PedsQL 4.0 Generic Core Scales (physical, emotional, social, and school functioning) and the 5 PedsQL 3.0 Rheumatology Module scales (pain and hurt, daily activities, treatment, worry, and communication) were administered to 231 children and 244 parents (271 subjects accrued overall) recruited from a pediatric rheumatology clinic. Results. Internal consistency reliability for the PedsQL Generic Core total scale score (␣ ⴝ 0.91 for child self report, ␣ ⴝ 0.93 for parent proxy report),
Health-related quality-of-life (HRQOL) measurement has increasingly been integrated into clinical trials, clinical practice improvement initiatives, and health care services research and evaluation as an essential health outcome (1–3). While health status, functional status, and HRQOL are terms often used interchangeably, a recent meta-analysis suggests that a more parsimonious distinction among these terms is indicated (4). Specifically, health status and functional status refer to the physical functioning dimensions of the broader HRQOL construct. HRQOL additionally includes the psychosocial dimensions of emotional, social, and role functioning, as well as related constructs. Thus, a pediatric HRQOL instrument must be multidimensional, consisting, at a minimum, of the physical, mental, and social health dimensions delineated by the World Health Organization (5). While the importance of measuring
Supported by intramural grants from Children’s Hospital and Health Center, San Diego, California, and by research grants from the Arthritis Foundation. 1 James W. Varni, PhD, Ilona S. Szer, MD: Children’s Hospital and Health Center, San Diego, California, and University of California, San Diego School of Medicine; 2Michael Seid, PhD, Tara Smith Knight, PhD, Tasha Burwinkle, MA, Joy Brown, BA: Children’s Hospital and Health Center, San Diego, California. Address correspondence and reprint requests to James W. Varni, PhD, Professor and Senior Scientist, Center for Child Health Outcomes, Children’s Hospital and Health Center, 3020 Children’s Way, San Diego, CA 92123. E-mail:
[email protected]. Submitted for publication March 22, 2001; accepted in revised form October 9, 2001. 714
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pain (6) and functional status (7–11) in pediatric rheumatology is now well accepted, the HRQOL measurement construct in pediatric rheumatology is a more contemporary conceptualization (12). Pediatric HRQOL measurement instruments must be sensitive to cognitive development and include child self report and parent proxy report to reflect their potentially unique perspectives. Imperfect concordance, termed cross-informant variance (13), has been consistently documented in the HRQOL assessment of children with chronic health conditions (14–18) and healthy children (19). Agreement has been found to be lower for internalizing problems (e.g., depression, pain) than for externalizing problems (e.g., hyperactivity, walking). Given that HRQOL derives from an individual’s perceptions, the demonstration of cross-informant variance indicates an essential need in pediatric HRQOL measurement for reliable and valid child self-report instruments for the broadest age range possible. The Pediatric Quality of Life Inventory (PedsQL; available online at http://www.pedsql.org) builds on a programmatic measurement instrument development effort by Varni and colleagues in pediatric chronic health conditions, including rheumatic diseases (8,20,21), during the past 15 years. The PedsQL 1.0 (22), originally derived from a pediatric cancer database (18,23,24), was designed as a generic HRQOL instrument to be utilized noncategorically, that is, across diverse pediatric populations. The PedsQL 2.0 and 3.0 included additional constructs and items, a more sensitive scaling range, and a broader age range for child self report and parent proxy report. The PedsQL 4.0 Generic Core Scales have resulted from this iterative process and include child self report for ages 5–18 and parent proxy report for ages 2–18. The PedsQL 4.0 Generic Core Scales were designed to measure the core health dimensions as delineated by the World Health Organization (5), as well as role (school) functioning. In the initial field trial, the PedsQL 4.0 Generic Core Scales were administered to 963 children and 1,629 parents (1,677 subjects accrued overall) (25). Internal consistency reliability for the total scale score (␣ ⫽ 0.88 for child self report, ␣ ⫽ 0.90 for parent proxy report), physical health summary score (␣ ⫽ 0.80 for child self report, ␣ ⫽ 0.88 for parent proxy report), and psychosocial health summary score (␣ ⫽ 0.83 for child self report, ␣ ⫽ 0.86 for parent proxy report) were acceptable for group comparisons. The PedsQL 4.0 Generic Core Scales distinguished between healthy children and pediatric patients with acute or chronic health conditions, were related to indicators of
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morbidity and illness burden, and displayed a factorderived solution largely consistent with the a priori conceptually derived scales (25). The PedsQL 3.0 Rheumatology Module was designed to measure HRQOL dimensions specifically tailored for pediatric rheumatology. While the relative merits of a generic versus disease-specific approach to measuring HRQOL is a matter of empirical inquiry (26), there are advantages to an integrated modular approach (22,27). Disease-specific modules may enhance measurement sensitivity for health domains germane to a particular chronic health condition. A generic HRQOL measurement instrument enables comparisons across pediatric populations and facilitates benchmarking with healthy populations. The current report presents the measurement properties of the PedsQL in pediatric rheumatology, addressing reliability, validity, and responsiveness in a diverse sample of children with rheumatic diseases. SUBJECTS AND METHODS Subjects and settings. Subjects were children ages 5–18 (n ⫽ 231) and parents of children ages 2–18 (n ⫽ 244), with 271 participants accrued overall. For 204 children ages 5–18, both child self report and parent proxy report were available. Subjects were recruited from the pediatric rheumatology clinic at Children’s Hospital and Health Center, San Diego. The PedsQL was self administered by parents and by children ages 8–18 and administered by an interviewer to children ages 5–7. The measures were administered in two languages—English (97.1%; n ⫽ 263) and Spanish (2.9%; n ⫽ 8). For all forms combined, the mean ⫾ SD age of the 63 boys (23.2%) and 207 girls (76.1%) was 11.47 ⫾ 4.27 years (range 2.38–18.90 years). Information on the sex of 2 children (0.7%) was missing. For child self report, the mean ⫾ SD age of the 51 boys (22.0%) and 181 girls (78.0%) was 12.43 ⫾ 3.59 years (range 5.01–18.90 years). The sample was heterogeneous with respect to race/ethnicity, with 127 white non-Hispanics (46.7%), 58 Hispanics (21.3%), 10 African American nonHispanics (3.7%), 11 Asians or Pacific Islanders (4.0%), 4 American Indians or Alaskan Natives (1.5%), 4 in the category of Other (1.5%), and 58 with missing information (21.3%). Mean socioeconomic status (SES) was 45.26, based on the Hollingshead index, indicating on average a middle-class– family SES (28). The sample included 81 children (29.8%) with juvenile rheumatoid arthritis (JRA), of whom 32 had the pauciarticular subtype (11.8%), 35 had the polyarticular subtype (12.9%), and 14 had the systemic subtype (5.1%); 22 children (8.1%) with systemic lupus erythematosus; 35 (12.9%) with juvenile fibromyalgia; 29 (10.7%) with spondylarthritis; and 105 (38.6%) with other rheumatic diseases. Patients and parents completed the PedsQL 4.0 Generic Core Scales and PedsQL 3.0 Rheumatology Module during rheumatology clinic visits. The mean ⫾ SD number of visits per patient was 3.05 ⫾ 2.33
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(range 1–13). Data for all patients are reported for the first administration of the PedsQL. The responsiveness data reflect HRQOL scores over time for new patients only, from visit 1 at the initial evaluation (prior to intervention) through visit 3. There was a mean ⫾ SD of 81.38 ⫾ 60.36 days between visits 1 and 2 and a mean ⫾ SD of 70.74 ⫾ 42.06 days between visits 2 and 3. Measures. The PedsQL 4.0 Generic Core Scales. The 23-item multidimensional PedsQL 4.0 Generic Core Scales encompass the following: 1) physical functioning scale (8 items), 2) emotional functioning scale (5 items), 3) social functioning scale (5 items), and 4) school functioning scale (5 items). The PedsQL 4.0 Generic Core Scales comprise parallel child self-report and parent proxy-report formats. Child self report includes ages 5–7 (young child), 8–12 (child), and 13–18 (adolescent). Parent proxy report includes ages 2–4 (toddler), 5–7 (young child), 8–12 (child), and 13–18 (adolescent). The parent proxy-report forms are parallel to the child self-report forms and are designed to assess the parent’s perceptions of the child’s HRQOL. The items for each of the forms are essentially identical, differing in developmentally appropriate language or in first- versus third-person tense. The instructions ask how much of a problem each item has been during the past 1 month. A 5-point Likert scale is utilized across child self report for ages 8–18 and parent proxy report (0 ⫽ never a problem, 1 ⫽ almost never a problem, 2 ⫽ sometimes a problem, 3 ⫽ often a problem, 4 ⫽ almost always a problem). To further increase the ease of use for the young child self report (ages 5–7), the Likert scale is reworded and simplified to a 3-point scale (0 ⫽ not at all a problem, 2 ⫽ sometimes a problem, 4 ⫽ a lot of a problem), with each response choice anchored to a happy-tosad–faces scale (20,21). Parent proxy report also includes the toddler age range (2–4 years), which does not include a self-report form, given the developmental limitations on self report for children younger than age 5 (21,29), and includes only 3 items for the school functioning scale. Items are reverse scored and linearly transformed to a 0–100 scale (0 ⫽ 100, 1 ⫽ 75, 2 ⫽ 50, 3 ⫽ 25, 4 ⫽ 0), so that higher PedsQL 4.0 scores indicate better HRQOL. This direct linear transformation does not affect the measurement properties of the scales and is computed for ease of interpretation so that scores near 0 indicate poorer HRQOL and scores near 100 indicate better HRQOL. Scale scores are computed as the sum of the items divided by the number of items answered (this accounts for missing data). If ⬎50% of the items in the scale are missing, the scale score is not computed. Imputing the mean of the completed items in a scale when ⱖ50% are completed is generally the most unbiased and precise method (30). For this study, ⬎99% of child and parent respondents were included in the scale score analyses. The physical health summary score (8 items) is the same as the physical functioning scale. To create the psychosocial health summary score (15 items), the mean is computed as the sum of the items divided by the number of items answered in the emotional, social, and school functioning scales. PedsQL 3.0 Rheumatology Module. The 22-item multidimensional PedsQL 3.0 Rheumatology Module encompasses the following scales: 1) pain and hurt (4 items), 2) daily activities (5 items), 3) treatment (7 items), 4) worry (3 items),
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and 5) communication (3 items). The format, instructions, Likert scale, and scoring method are identical to those of the PedsQL 4.0 Generic Core Scales, with higher scores indicating better HRQOL (fewer problems or symptoms). The PedsQL 3.0 Rheumatology Module was developed based on our research and clinical experiences in pediatric rheumatology and on the instrument development literature (31–33), which consists of a review of the extant literature, patient and parent focus groups and individual focus interviews, item generation, cognitive interviews, and pretesting and subsequent field testing of the new measurement instrument in the target population. Our development of the PedsQL 3.0 Rheumatology Module was also informed by our group’s instrument development research with both the Pediatric Pain Questionnaire (20,21,34,35) and our group’s arthritis-specific functional status measure (8), as well as with earlier iterations of the PedsQL that were pretested in pediatric rheumatology. Parent proxy report for the toddler age range (2–4 years) does not include the worry and communication scales, since the cognitive interviews indicated that parents were not able to ascertain these constructs in children at this developmental stage. PedsQL family information form. The PedsQL family information form, completed by parents, contains demographic information about the child and parents. It contains the information required to calculate the Hollingshead SES index (28). Procedure. Potential subjects were identified through the clinic appointment schedule. The parents of the children identified as possible study participants were informed of the study after checking in for their appointment, but before being seen by their health care provider. Written parental informed consent and child assent were obtained. Parents and children completed the PedsQL separately. One parent (65.8% mothers, 13.2% fathers, 3.7% other, 17.3% missing) completed the proxy-report version. A research assistant or specifically trained clinic personnel was available to answer questions regarding the parent self-administered instruments. A research assistant or specifically trained clinic personnel administered the PedsQL for the young child (ages 5–7) and was available to assist the child (ages 8–12) and adolescent (ages 13–18) with the self-administered instrument after the instructions had been given and clarified. The average time needed to complete both the PedsQL 4.0 Generic Core Scales and the PedsQL 3.0 Rheumatology Module is estimated to be 15 minutes for child self report and 10 minutes for parent proxy report, based on our experience administering these measures in the rheumatology clinic. Previous research indicated that it takes ⬃4 minutes to administer the PedsQL 4.0 Generic Core Scales (25). The research protocol was approved by the Institutional Review Board at Children’s Hospital and Health Center. Statistical analysis. Feasibility or practicality was determined from the percentage of missing values (36). Scale internal consistency reliability was determined by calculating Cronbach’s coefficient alpha (37). Scales with reliabilities of ⱖ0.70 are recommended for comparing patient groups, while a reliability criterion of 0.90 is recommended for analyzing individual patient scale scores (38,39). Construct validity was determined utilizing the knowngroups method. The known-groups method compares scale
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Table 1. Scale descriptives for Pediatric Quality of Life Inventory 4.0 Generic Core Scales for child self report and parent proxy report, and between-group comparisons* Children with rheumatic diseases Scale Self report Total score Physical health Psychosocial health Emotional functioning Social functioning School functioning Proxy report Total score Physical health Psychosocial health Emotional functioning Social functioning School functioning
Healthy children
No. of items
No. of subjects
Mean
SD
No. of subjects
Mean
SD
Difference
t†
23 8 15 5 5 5
231 231 231 231 231 227
72.09 68.12 74.23 70.88 80.75 71.39
16.92 22.52 16.46 22.08 18.27 18.86
401 400 399 400 399 386
83.00 84.41 82.38 80.86 87.42 78.63
14.79 17.26 15.51 19.64 17.18 20.53
10.91 16.29 8.15 9.98 6.67 7.24
8.46 10.18 6.21 5.87 4.59 4.34
23 8 15 5 5 5
244 244 244 242 242 229
70.97 66.72 73.31 69.00 77.49 73.40
18.49 24.12 17.62 21.39 20.06 21.07
717 717 717 718 716 611
87.61 89.32 86.58 82.64 91.56 85.47
12.33 16.35 12.79 17.54 14.20 17.61
16.64 22.59 13.27 13.64 14.07 12.07
15.84 16.35 12.60 9.88 11.85 8.37
* Children with rheumatic diseases as a group were compared with the healthy children population group from a previous field trial (see ref. 25). † All scale scores for the group of children with rheumatic diseases are significantly different from those for the group of healthy children (P ⱕ 0.001).
scores across groups known to differ in the health construct being investigated. In this study, PedsQL 4.0 Generic Core Scales scores in groups differing in known health condition (healthy children and children with rheumatic diseases) were computed (40,41) using one-way analysis of variance (ANOVA). The data for the healthy group of children were derived from the initial field trial of the PedsQL 4.0 Generic Core Scales (25). We hypothesized that healthy children would report higher PedsQL 4.0 scores (better HRQOL) than pediatric patients with rheumatic diseases. Exploratory analyses were conducted to examine whether there were differences in PedsQL Generic Core Scales scores and Rheumatology Module scale scores among children with different rheumatic diseases. Construct validity was further examined through an analysis of the intercorrelations among the PedsQL 4.0 Generic Core total scale score and the PedsQL 3.0 Rheumatology Module scale scores. It was hypothesized that higher module scores (fewer problems or symptoms) would be correlated with higher Generic Core total scale scores (better overall HRQOL), based on the conceptualization of disease-specific symptoms as causal indicators of HRQOL (1). Correlation effect sizes are designated as small (0.10–0.29), medium (0.30– 0.49), and large (ⱖ0.50) (42). Intercorrelations were expected to demonstrate medium-to-large effect sizes (1). The responsiveness of the PedsQL was determined through individual patient changes over time (1). The responsiveness of a measurement instrument is demonstrated through a longitudinal analysis of changes within patients in whom a change is anticipated as a result of, for example, an intervention of known or expected efficacy (1). In order to determine the magnitude of change, effect sizes were calculated (42,43). Effect size as utilized in these analyses was
calculated by taking the difference between the time-1 mean and the time-2 mean, divided by the time-1 SD, and by taking the difference between the time-1 mean and the time-3 mean, divided by the time-1 SD. Effect sizes are designated as small (0.20), medium (0.50), and large (0.80) in magnitude of change (42). The intervention for the present investigation was the clinical care provided by the director of the Division of Pediatric Rheumatology (ISS). For the purposes of the present investigation, the responsivenesses of individual patient change over time for the PedsQL summary and scales scores were calculated through a repeated-measures ANOVA (38,39). The PedsQL 4.0 Generic Core Scales total and summary scales scores were selected, given that their reliability (alpha) coefficients meet or approach 0.90, which is recommended for analyzing individual patient scale scores (38,39). For the purposes of these analyses, we computed scales scores for the responsiveness analysis of the Rheumatology Module scales, since the module does not have summary scores. Parent–child intercorrelations were computed to examine cross-informant variance. Correlation effect sizes are designated as small (0.10–0.29), medium (0.30–0.49), and large (ⱖ0.50) (42). Parent–child concordance for the total score, summary scores, and the same scale were expected to demonstrate medium-to-large effect sizes, but not so large that child and parent reports would be redundant. Statistical analyses were conducted using SPSS for Windows (SPSS, Chicago, IL) (44). Response equivalence has been previously demonstrated across languages (English versus Spanish) for the PedsQL by examining the percent missing data, floor, and ceiling effects and scale internal consistency across languages (25). Therefore, responses were pooled across languages. Responses were also pooled across the age ranges for both self report and proxy report.
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Table 2. Scale descriptives for Pediatric Quality of Life Inventory 3.0 Rheumatology Module for child self report and parent proxy report Scale Self report Pain and hurt Daily activities Treatment Worry Communication Proxy report Pain and hurt Daily activities Treatment Worry Communication
No. of items
No. of subjects
Mean
SD
4 5 7 3 3
231 231 230 213 231
61.87 90.09 77.97 74.16 70.53
28.31 15.00 19.62 25.17 26.94
4 5 7 3 3
242 241 241 232 232
61.93 86.30 74.58 78.70 72.22
28.25 22.11 20.28 23.69 27.58
RESULTS Missing item responses. To assess the feasibility or practicality of administration for the PedsQL 4.0 Generic Core Scales, the percentage of missing values was calculated. For child self report and parent proxy
report, the percentages of missing item responses were 0.7% and 3%, respectively. The percentage of missing items for the school functioning scale was higher than that for the other scales, reflecting in part that the items on the school functioning scale were not completed when the child did not attend school during the previous month (e.g., data were collected during summer vacation as well as at other times throughout the year). For the PedsQL 3.0 Rheumatology Module, the percentage of missing item responses was 2% for child self report and 2% for parent proxy report. Means and SDs. Table 1 presents the means and SDs of the PedsQL 4.0 Generic Core Scales scores for children with rheumatic diseases as a group and for the healthy children population group from our previous field trial (25). Table 2 presents the means and SDs of the PedsQL 3.0 Rheumatology Module scale scores for the pediatric rheumatology group. Internal consistency reliability. Internal consistency reliability alpha coefficients for the PedsQL 4.0 Generic Core Scales and the PedsQL 3.0 Rheumatology
Table 3. Pediatric Quality of Life Inventory (PedsQL) 4.0 Generic Core Scales and PedsQL 3.0 Rheumatology Module internal consistency reliability for child self report and parent proxy report by age and summary or scale score* Subject category (age range, years) Scale Generic Core Scales Self report Total score Physical health Psychosocial health Emotional functioning Social functioning School functioning Proxy report Total score Physical health Psychosocial health Emotional functioning Social functioning School functioning Rheumatology Module Self report Pain and hurt Daily activities Treatment Worry Communication Proxy report Pain and hurt Daily activities Treatment Worry Communication
Toddler (2–4)
Young child (5–7)
Child (8–12)
Adolescent (13–18)
Total sample
NA NA NA NA NA NA
0.85 0.63 0.81 0.73 0.58 0.48
0.92 0.89 0.87 0.78 0.79 0.74
0.92 0.90 0.87 0.82 0.78 0.74
0.91 0.87 0.86 0.79 0.75 0.71
0.77 0.75 0.76 0.79 0.67 NA
0.93 0.88 0.89 0.85 0.73 0.78
0.94 0.91 0.90 0.86 0.76 0.79
0.94 0.88 0.91 0.84 0.84 0.81
0.93 0.89 0.90 0.84 0.80 0.79
NA NA NA NA NA
0.56 0.55 0.75 0.55 0.67
0.87 0.72 0.80 0.67 0.85
0.90 0.84 0.77 0.81 0.79
0.86 0.78 0.80 0.75 0.78
0.85 0.88 0.59 NA NA
0.90 0.91 0.81 0.89 0.87
0.91 0.93 0.85 0.78 0.90
0.90 0.89 0.77 0.80 0.91
0.91 0.91 0.82 0.83 0.89
* Values are Cronbach’s coefficient alpha. NA ⫽ not applicable.
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Table 4. Item score distributions for the Pediatric Quality of Life Inventory (PedsQL) 4.0 Generic Core Scales and the PedsQL 3.0 Rheumatology Module for child self report and parent proxy report* Responses Scale Generic Core Scales Self report Total score Physical health Psychosocial health Emotional functioning Social functioning School functioning Proxy report Total score Physical health Psychosocial health Emotional functioning Social functioning School functioning Rheumatology Module Self report Pain and hurt Daily activities Treatment Worry Communication Proxy report Pain and hurt Daily activities Treatment Worry Communication
Never
Almost never
Sometimes
Often
Almost always
Missing
44.23 43.13 44.82 40.61 56.54 37.32
18.84 14.29 21.27 21.13 18.70 23.98
23.38 23.59 23.26 24.76 18.10 26.93
6.46 8.82 5.19 6.93 3.64 5.02
6.40 9.74 4.62 6.15 2.77 4.94
0.70 0.43 0.84 0.43 0.26 1.82†
40.39 40.27 40.46 32.21 49.67 39.43
20.57 15.63 23.25 26.39 21.97 21.31
23.06 22.59 23.31 28.36 17.95 23.63
8.62 11.17 7.24 8.85 6.07 6.79
5.47 9.32 3.39 3.36 3.20 3.61
1.89 1.02 2.36 0.82 1.15 5.24†
33.87 77.14 55.15 45.60 43.87
15.91 9.70 14.08 15.15 17.46
25.97 9.26 15.38 18.61 21.93
10.82 1.82 3.91 6.20 6.64
12.88 1.39 5.80 6.06 8.66
0.54 0.69 5.67‡ 8.37‡ 1.44
30.33 71.48 47.56 50.77 44.13
19.36 10.49 16.94 16.82 21.30
25.72 9.34 19.26 19.14 21.91
14.04 2.87 7.19 6.48 8.18
9.43 3.93 4.75 3.24 5.40
1.13 1.89 4.30 3.55 1.08
* Values are percentages. † For the PedsQL 4.0 Generic Core Scales, there is a higher percentage of missing values for the school functioning scale (see explanation in Results). Because the school functioning scale is calculated into the psychosocial health scale score and the total score, the percentages of missing values in the psychosocial health scale score and the total score are likewise affected. ‡ For the treatment and worry scales (self report), the higher percentage of missing values is due primarily to the young-child version.
Module across all ages are presented in Table 3. The majority of child self-report scales and parent proxyreport scales approached or exceeded the minimum reliability standard of 0.70 for group comparisons (38). The PedsQL 4.0 Generic Core Scales total score for ages 5–18 approached or exceeded the reliability criterion of 0.90 recommended for analyzing individual patient scores (38,39). Range of responses. Table 4 includes the percentage of responses in each response category and the percentage of missing values for both self-report and proxy-report versions of the PedsQL 4.0 Generic Core Scales and the PedsQL 3.0 Rheumatology Module. Construct validity. Table 1 demonstrates the comparisons between the PedsQL 4.0 Generic Core Scales scores for the healthy children group and those for children with rheumatic diseases as a group. For every comparison, there was a statistically significant difference between healthy children and children with
rheumatic diseases. The hypothesis that healthy children as a group would manifest higher PedsQL 4.0 Generic Core Scales scores than would children with rheumatic diseases as a group was thus confirmed. Tables 5 and 6 display the one-way ANOVAs comparing children with different rheumatic diseases and healthy children. For child self report, the PedsQL 4.0 Generic Core Scales total score demonstrated significant differences between the healthy population group and children with JRA or fibromyalgia (Table 5). For parent proxy report, the PedsQL 4.0 Generic Core Scales total score demonstrated significant differences between the healthy population group and all pediatric rheumatic disease groups (Table 6). The exploratory analyses among children with different rheumatic diseases revealed that the most consistent group differences were observed primarily between children with fibromyalgia and the other groups of children with rheumatic diseases (Tables 5 and 6). For the PedsQL 3.0
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Rheumatology Module scales, the exploratory analyses among children with different rheumatic diseases revealed that for child self report, group differences were
Table 6. One-way analyses of variance comparing known groups for the Pediatric Quality of Life Inventory 4.0 Generic Core Scales parent proxy report* Scale, group
Table 5. One-way analyses of variance comparing known groups for the Pediatric Quality of Life Inventory 4.0 Generic Core Scales child self report* Scale, group Total score† DM FM JRA SLE SpA H Physical health† DM FM JRA SLE SpA H Psychosocial health† DM FM JRA SLE SpA H Emotional functioning† DM FM JRA SLE SpA H Social functioning† DM FM JRA SLE SpA H School functioning† DM FM JRA SLE SpA H
No. of subjects Mean
SD
14 33 62 22 29 401
76.32 57.25 74.74 75.85 75.72 83.00
17.67 15.48 15.53 14.85 17.17 14.79
14 33 62 22 29 400
74.11 46.78 72.12 75.95 69.27 84.41
20.12 19.54 20.57 20.70 25.36 17.26
14 33 62 22 29 399
77.50 62.84 76.09 75.96 79.15 82.38
17.55 17.24 15.90 15.84 15.31 15.51
14 33 62 22 29 400
78.93 55.15 73.75 67.67 79.87 80.86
16.31 24.19 20.56 24.85 20.07 19.64
14 33 62 22 29 399
74.29 74.85 80.16 89.26 84.83 87.42
23.77 18.94 16.91 14.25 17.95 17.18
14 33 61 20 29 386
79.29 58.52 74.75 71.75 72.76 78.63
17.19 20.64 18.56 16.00 20.77 20.53
df
F
P
5, 555 20.53 0.001
5, 554 30.48 0.001
5, 553 10.84 0.001
5, 554 11.80 0.001
5, 553
5, 537
6.00 0.001
6.70 0.001
* df ⫽ degrees of freedom; DM ⫽ dermatomyositis; FM ⫽ fibromyalgia; JRA ⫽ juvenile rheumatoid arthritis (includes pauciarticular, polyarticular, and systemic subtypes); SLE ⫽ systemic lupus erythematosus; SpA ⫽ spondylarthritis; H ⫽ healthy. † The following between-group differences were significant, based on Tukey highest significant difference post-hoc analysis: for total score, JRA ⬍ H (P ⱕ 0.01), FM ⬍ DM (P ⱕ 0.01), FM ⬍ JRA, SLE, SpA, H (P ⱕ 0.001); for physical health, JRA, SpA ⬍ H (P ⱕ 0.001), FM ⬍ DM, JRA, SLE, SpA, H (P ⱕ 0.001); for psychosocial health, FM ⬍ DM, SLE (P ⱕ 0.05), FM ⬍ JRA, SpA, H (P ⱕ 0.01), JRA ⬍ H (P ⱕ 0.05); for emotional functioning, FM ⬍ DM, JRA, SpA, H (P ⱕ 0.01); for social functioning, FM ⬍ SLE (P ⱕ 0.05), FM ⬍ H (P ⱕ 0.01), JRA ⬍ H (P ⱕ 0.05); for school functioning, FM ⬍ DM (P ⱕ 0.05), FM ⬍ JRA, H (P ⱕ 0.01).
Total score† DM FM JRA SLE SpA H Physical health† DM FM JRA SLE SpA H Psychosocial health† DM FM JRA SLE SpA H Emotional functioning† DM FM JRA SLE SpA H Social functioning† DM FM JRA SLE SpA H School functioning† DM FM JRA SLE SpA H
No. of subjects Mean
SD
16 34 76 12 24 717
71.59 51.97 74.99 73.89 74.06 87.61
18.28 16.06 17.54 20.36 15.87 12.33
16 34 76 12 24 717
70.31 43.41 71.93 70.35 70.31 89.32
20.12 19.63 21.89 28.79 25.77 16.35
16 34 76 12 24 717
72.40 56.53 76.63 76.18 76.01 86.58
18.60 16.80 17.14 17.96 13.62 12.79
16 34 76 12 24 718
67.50 47.50 75.89 69.17 77.50 82.64
21.83 18.72 18.95 24.94 14.74 17.54
16 34 76 12 23 716
74.06 66.03 76.78 81.67 80.22 91.56
23.18 20.33 19.30 18.87 18.62 14.20
14 34 70 11 24 611
75.89 56.07 76.97 78.64 70.83 85.47
19.26 21.39 21.53 19.63 17.86 17.61
df
F
P
5, 873 63.16 0.001
5, 873 62.19 0.001
5, 873 42.13 0.001
5, 874 29.07 0.001
5, 871 33.77 0.001
5, 758 21.23 0.001
* See Table 5 for definitions. † The following between-group differences were significant, based on Tukey highest significant difference post-hoc analysis: for total score, DM, FM, JRA, SpA ⬍ H (P ⱕ 0.001), SLE ⬍ H (P ⱕ 0.05), FM ⬍ DM, JRA, SLE, SpA (P ⱕ 0.001); for physical health, DM, FM, JRA, SpA ⬍ H (P ⱕ 0.001), SLE ⬍ H (P ⱕ 0.01), FM ⬍ DM, JRA, SLE, SpA (P ⱕ 0.001); for psychosocial health, DM, FM, JRA, SpA ⬍ H (P ⱕ 0.05), FM ⬍ DM, JRA, SLE, SpA (P ⱕ 0.05); for emotional functioning, DM, JRA ⬍ H (P ⱕ 0.05), FM ⬍ DM, JRA, SLE, SpA, H (P ⱕ 0.01); for social functioning, FM ⬍ JRA, SLE, SpA (P ⱕ 0.05), DM, FM, JRA, SpA ⬍ H (P ⱕ 0.01); for school functioning, FM ⬍ DM, JRA, SLE, SpA (P ⱕ 0.05), FM, JRA, SpA ⬍ H (P ⱕ 0.01).
observed on the pain and hurt scale between children with fibromyalgia and the other groups of children with rheumatic diseases, as well as between children with spondylarthritis and those with systemic lupus erythem-
THE PEDSQL
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Table 7. One-way analyses of variance comparing known groups for the Rheumatology Module child self report* Scale, group Pain and hurt† DM FM JRA SLE SpA Daily activities DM FM JRA SLE SpA Treatment‡ DM FM JRA SLE SpA Worry DM FM JRA SLE SpA Communication§ DM FM JRA SLE SpA
No. of subjects
Mean
SD
14 33 62 22 29
77.68 34.34 68.35 76.42 55.60
16.76 27.48 23.63 20.86 31.80
14 33 62 22 29
95.00 85.61 87.70 96.14 94.66
7.84 18.61 16.03 8.58 7.43
14 33 62 22 29
84.09 74.31 76.93 88.99 82.33
15.08 19.97 19.31 12.61 17.60
13 33 58 21 29
74.04 72.22 73.42 65.67 82.47
27.12 27.14 23.39 33.90 18.94
14 33 62 22 29
75.60 61.36 68.68 75.38 81.03
20.27 25.41 24.41 28.00 26.62
df
F
P
4, 155
14.35
0.000
4, 155
3.48
0.009
4, 155
2.92
0.023
4, 149
1.41
0.233
4, 155
2.76
0.030
* See Table 5 for other definitions. † According to Tukey highest significant difference (HSD) post-hoc analysis, for the pain and hurt scale only, FM differs significantly from all other diagnostic groups (P ⱕ 0.05), and SpA differs significantly from SLE (P ⱕ 0.05). ‡ According to Tukey HSD post-hoc analysis, for the treatment scale only, FM differs significantly from SLE (P ⱕ 0.05). § According to Tukey HSD post-hoc analysis, for the communication scale only, FM differs significantly from SpA (P ⱕ 0.05).
atosus (Table 7). In addition, children with fibromyalgia differed significantly from those with systemic lupus erythematosus (on the treatment scale only) as well as from those with spondylarthritis (on the communication scale only) (Table 7). For parent proxy report, group differences were observed on the pain and hurt scale between children with fibromyalgia and the other groups of children with rheumatic diseases, between children with fibromyalgia and those with systemic lupus erythematosus (on the treatment scale only), and between children with fibromyalgia and those with JRA or spondylarthritis (on the worry scale only) (Table 8). The intercorrelations between the PedsQL 4.0 Generic Core Scales total score and the PedsQL 3.0 Rheumatology Module scales are shown in Table 9. As anticipated, the correlations were in the medium-to-
large effect size range, and the largest intercorrelations were those of the pain and hurt scale with the total scale score for both child and parent report. The intercorrelations between all the Generic Core Scales and the Rheumatology Module scales were also in the mediumto-large effect size range. Parent–child concordance. The parent–child concordance intercorrelations matrix is shown in Table 9. Consistent with the extant literature, child self-report and parent proxy-report correlations were in the medium-to-large effect size range. Responsiveness. The responsiveness of the PedsQL 4.0 Generic Core Scales total scale score and physical health and psychosocial health summary scores is shown in Table 10. Table 10 also contains the PedsQL 3.0 Rheumatology Module responsiveness data. These
Table 8. One-way analyses of variance comparing known groups for the Rheumatology Module parent proxy report* Scale, group Pain and hurt† DM FM JRA SLE SpA Daily activities DM FM JRA SLE SpA Treatment‡ DM FM JRA SLE SpA Worry§ DM FM JRA SLE SpA Communication DM FM JRA SLE SpA
No. of subjects
Mean
SD
16 34 76 11 24
78.91 33.64 69.55 78.98 57.55
15.95 27.05 23.00 20.40 30.28
16 34 75 12 24
81.56 81.86 84.13 88.13 88.33
29.02 21.00 24.57 28.15 20.36
16 33 75 12 24
81.76 66.69 75.43 85.12 77.09
19.25 20.45 21.32 15.14 16.84
16 33 74 12 24
72.92 66.67 82.71 72.92 85.42
26.61 27.42 22.28 31.81 18.43
16 34 73 12 24
70.31 67.16 75.06 72.92 77.78
30.88 27.37 26.90 32.20 22.34
df
F
P
4, 156
16.74
0.000
4, 156
0.385
0.819
4, 155
2.735
0.031
4, 154
3.396
0.011
4, 154
0.719
0.580
* See Table 5 for definitions. † According to Tukey highest significant difference (HSD) post-hoc analysis, for the pain and hurt scale only, FM differs significantly from all other diagnostic groups (P ⱕ 0.05). ‡ According to Tukey HSD post-hoc analysis, for the treatment scale only, FM differs significantly from SLE (P ⱕ 0.05). § According to Tukey HSD post-hoc analysis, for the worry scale only, FM differs significantly from JRA and SpA (P ⱕ 0.05).
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Table 9. Pearson correlation coefficients among Pediatric Quality of Life Inventory scales* Pain Total Physical Psychosocial Emotional Social School and Daily score health health functioning functioning functioning hurt activities Treatment Worry Communication Total score Physical health Psychosocial health Emotional functioning Social functioning School functioning Pain and hurt Daily activities Treatment Worry Communication
0.654 0.904 0.946 0.789 0.823 0.809 0.688 0.462 0.495 0.530 0.456
0.876 0.710 0.716 0.582 0.655 0.601 0.722 0.355 0.408 0.449 0.368
0.935 0.649 0.552 0.847 0.846 0.860 0.580 0.487 0.496 0.524 0.468
0.785 0.512 0.862 0.514 0.552 0.590 0.549 0.326 0.459 0.557 0.413
0.753 0.540 0.794 0.521 0.499 0.620 0.444 0.422 0.370 0.365 0.381
0.776 0.553 0.816 0.550 0.472 0.511 0.520 0.507 0.457 0.419 0.405
0.688 0.757 0.529 0.433 0.371 0.503 0.744 0.350 0.486 0.421 0.310
0.546 0.504 0.490 0.379 0.445 0.371 0.473 0.503 0.431 0.335 0.328
0.506 0.442 0.473 0.385 0.407 0.361 0.435 0.474 0.558 0.435 0.320
0.460 0.391 0.438 0.434 0.306 0.301 0.307 0.423 0.439 0.482 0.343
0.493 0.410 0.476 0.400 0.390 0.372 0.364 0.339 0.451 0.425 0.285
* Correlations for patient self report are shown above the diagonal; underlined values represent correlations between patient self report and parent proxy report. Correlations for parent proxy report are shown below the diagonal. Correlations among Generic Core Scales and Rheumatology Module scales are in boldface. All correlations are significant (P ⱕ 0.01, by 2-tailed Pearson product-moment correlation). Effect sizes are designated as small (0.10–0.29), medium (0.30–0.49), and large (ⱖ0.50).
Table 10. Responsiveness of the Pediatric Quality of Life Inventory (PedsQL) 4.0 Generic Core Scales and Rheumatology Module scales for new patients Change in PedsQL scores Visit 1
Generic Core Scales Child self report‡ Total score§ Physical health§ Psychosocial health§ Parent proxy report‡ Total score§ Physical health§ Psychosocial health Rheumatology Module Child self report‡ Pain and hurt§ Daily activities Treatment Worry§ Communication§ Parent proxy report‡ Pain and hurt§ Daily activities Treatment Worry Communication
Visit 2
Visit 3
Effect size†
Mean
SD
Mean
SD
Mean
SD
df*
F
P
Visit 1 – visit 2
Visit 1 – visit 3
68.35 61.65 72.04
20.20 27.31 19.29
75.24 72.49 76.69
19.81 23.44 19.23
87.02 89.06 85.93
12.05 11.85 13.73
2, 83 2, 83 2, 83
5.90 8.17 3.40
0.004 0.001 0.038
0.34 0.40 0.24
0.92 1.00 0.72
68.94 61.46 73.06
21.68 28.84 20.30
74.78 69.14 77.97
20.00 26.36 17.96
84.35 83.28 84.97
16.64 16.42 18.01
2, 83 2, 83 2, 83
3.71 4.57 2.47
0.029 0.013 0.090
0.27 0.27 0.24
0.71 0.76 0.59
51.47 90.00 73.76 72.58 64.95
29.03 15.13 22.05 24.39 30.28
68.20 93.82 73.61 80.81 75.61
30.63 9.85 23.11 18.22 21.96
86.11 97.08 83.39 94.44 90.74
16.82 4.22 18.41 10.31 16.88
2, 83 2, 83 2, 82 2, 79 2, 83
9.52 2.36 1.41 7.11 6.42
0.000 0.101 0.251 0.001 0.003
0.58 0.25 0.01 0.34 0.35
1.19 0.47 0.44 0.90 0.85
52.34 90.16 71.44 77.55 76.75
31.65 13.88 18.67 25.15 27.42
63.76 92.08 76.57 74.87 76.08
30.43 15.20 23.15 24.21 27.28
82.50 92.89 79.42 82.84 88.24
18.98 13.67 24.24 23.10 17.45
2, 83 2, 81 2, 80 2, 76 2, 76
6.86 0.26 0.88 0.59 1.43
0.001 0.768 0.420 0.557 0.246
0.36 0.14 0.27 0.27 0.02
0.95 0.20 0.43 0.21 0.42
* Degrees of freedom. † Designated as small (0.20), medium (0.50), and large (0.80). ‡ For visits 1, 2, and 3, respective numbers of patients were as follows: for Generic Core Scales child self report, n ⫽ 34, n ⫽ 34, and n ⫽ 18; for Generic Core Scales parent proxy report, n ⫽ 33, n ⫽ 33, and n ⫽ 20; for Rheumatology Module child self report, n ⫽ 34, n ⫽ 34, and n ⫽ 18; for Rheumatology Module parent proxy report, n ⫽ 33, n ⫽ 33, and n ⫽ 20. § The following between-visit differences in scores were significant, based on Tukey highest significant difference post-hoc analysis: for Generic Core Scales child self report total score, visit 1 ⬍ visit 3 (P ⱕ 0.01), physical health, visit 1 ⬍ visit 3 (P ⱕ 0.001) and visit 2 ⬍ visit 3 (P ⱕ 0.05), psychosocial health, visit 1 ⬍ visit 3 (P ⱕ 0.05); for Generic Core Scales parent proxy report total score, visit 1 ⬍ visit 3 (P ⱕ 0.05), physical health, visit 1 ⬍ visit 3 (P ⱕ 0.01); for Rheumatology Module child self report pain and hurt, visit 1 ⬍ visit 2 (P ⱕ 0.05) and visit 1 ⬍ visit 3 (P ⱕ 0.001), worry, visit 1 ⬍ visit 3 (P ⱕ 0.001), communication, visit 1 ⬍ visit 3 (P ⱕ 0.01); for Rheumatology Module parent proxy report pain and hurt, visit 1 ⬍ visit 3 (P ⱕ 0.001).
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data represent those clinic visits in which the pediatric rheumatologist evaluated the child for the first time (i.e., first visit as a new patient), with treatment then initiated at this visit (visit 1), with two subsequent followup visits (visits 2 and 3). Thirty-four children were assessed at visit 1; of those, 34 had a subsequent visit 2 and 18 had a visit 3. The 34 patients included in the responsiveness analysis had the following diagnoses: JRA (n ⫽ 7), fibromyalgia (n ⫽ 6), spondylarthritis (n ⫽ 9), dermatomyositis (n ⫽ 2), systemic lupus erythematosus (n ⫽ 2), and other rheumatic diseases (n ⫽ 8). For both child self report and parent proxy report, the PedsQL 4.0 Generic Core total and summary scale scores increased progressively from visit 1 through visit 3, with larger effect sizes at time 3. Effect sizes were in the small range at time 2 and in the medium-to-large effect size range at time 3. The PedsQL 3.0 Rheumatology Module scales demonstrated improvement over time primarily for pain and hurt, with small-to-medium effect sizes at time 2 and large effect sizes at time 3. DISCUSSION This report presents the measurement properties for the PedsQL 4.0 Generic Core Scales and PedsQL 3.0 Rheumatology Module in pediatric rheumatology. The analyses support the reliability, validity, and responsiveness of the PedsQL as a child self-report and parent proxy-report HRQOL measurement instrument for pediatric rheumatology. The PedsQL is the only empirically validated generic and rheumatology-specific HRQOL measurement instrument we are aware of that spans this broad age range for child self report and parent proxy report, while maintaining item and scale construct consistency. Items on the PedsQL had minimal missing responses, suggesting that children and parents are willing and able to provide good-quality data regarding the child’s HRQOL. The PedsQL self-report and proxyreport internal consistency reliabilities generally exceeded the recommended minimum alpha coefficient standard of 0.70 for group comparisons. For self report by children ages 8–18 and parent proxy report for children ages 5–18, the PedsQL 4.0 total scale score met or exceeded an alpha coefficient of 0.90, recommended for individual patient analysis (38), making the total scale score suitable as a summary score for the primary analysis of HRQOL outcome in clinical trials and other group comparisons for these ages. The physical health and psychosocial health summary scores for self report by children ages 8–18 and parent proxy report for
children ages 5–18 approached or met the ␣ ⫽ 0.90 criterion and are recommended for secondary analyses. The emotional, social, and school functioning scales’ alpha coefficients for the younger ages were more variable and should be used only for descriptive or exploratory analyses for younger children. The internal consistency reliabilities of the PedsQL 3.0 Rheumatology Module scales generally exceeded the recommended minimum alpha coefficient standard of 0.70 for group comparisons for self report by children ages 8–18 and for parent proxy report for children ages 2–18. The treatment scale for parent proxy report for children ages 2–4 and 3 scales for self report by children ages 5–7 had alpha coefficients of 0.55–0.59, suggesting that further empirical testing is necessary for these age groups. The PedsQL 4.0 Generic Core Scales performed as hypothesized utilizing the known-groups method. The PedsQL 4.0 differentiated HRQOL in healthy children as a group from that in children with rheumatic diseases as a group. The PedsQL 3.0 Rheumatology Module pain and hurt scale differentiated fibromyalgia from other pediatric rheumatic diseases, consistent with empirical reports in the literature (6). The intercorrelations among the PedsQL 4.0 Generic Core Scales and PedsQL 3.0 Rheumatology Module scales are consistent with the conceptualization of disease-specific symptoms as causal indicators of HRQOL (1). While other pediatric HRQOL instruments exist, including generic measures and rheumatology-specific measures, it has been an explicit goal of the PedsQL measurement model to develop and test brief measures for the broadest age group empirically feasible, specifically including child self report for the youngest children possible. This goal was originally determined in previous empirical efforts to measure pain in pediatric rheumatology through the development and testing of the Pediatric Pain Questionnaire (20). Thus, the development and testing of the PedsQL measurement model emphasizes the child’s perceptions, including those of the youngest children for whom this is empirically feasible. The items chosen for inclusion were initially derived from the measurement properties of the child self-report scales, while the parent proxy-report scales were constructed to directly parallel the child self-report items. The cross-informant variance observed in the parent–child intercorrelations matrix underscores the need to measure the potentially differing perspectives of child and parent informants in evaluating HRQOL in pediatric rheumatology. The use of parent proxy report to estimate child HRQOL may be necessary when the
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child is either unable or unwilling to complete the HRQOL measure. However, proxy reports should be conducted with the knowledge that proxy ratings of HRQOL may not be sufficiently accurate (45). Although it is likely that children and adolescents will have different concerns related to HRQOL, the items selected for the PedsQL 4.0 reflect areas of universal concern across age groups. Attempts were made to keep wording, and thus content, of items as similar as possible across parallel forms while being sensitive to developmental differences in cognitive ability. This consistency facilitates the evaluation of differences in HRQOL across and between age groups, as well as the longitudinal tracking of HRQOL. The present findings have several potential limitations. Information on nonparticipants was not available, and the field test was conducted in one children’s hospital, which may limit the generalizability of the findings. External measures, such as severity of illness, physician ratings of disease severity, and health care utilization, were not available for this study. These external measures would further contribute to the instrument’s construct validity. Additionally, the responsiveness calculations were based on only 34 children. A larger sample size would increase the generalizability of these responsiveness findings. However, instrument validation is an iterative process, and, consistent with this paradigm, the PedsQL scales are currently being further field tested nationally and internationally in pediatric rheumatology. The PedsQL measurement model represents a conceptual framework for a measurement instrument that must be at once disease specific and also reflective of broader generic concerns (46,47). REFERENCES 1. Fayers PM, Machin D. Quality of life: assessment, analysis, and interpretation. New York: Wiley; 2000. 2. Spilker B. Quality of life and pharmacoeconomics in clinical trials. Philadelphia: Lippincott-Raven; 1996. 3. Varni JW, Seid M, Kurtin PS. Pediatric health-related quality of life measurement technology: a guide for health care decision makers. J Clin Outcomes Meas 1999;6:33–40. 4. Smith KW, Avis NE, Assmann SF. Distinguishing between quality of life and health status in quality of life research: a meta-analysis. Qual Life Res 1999;8:447–59. 5. World Health Organization. Constitution of the World Health Organization basic document. Geneva (Switzerland): World Health Organization; 1948. 6. Varni JW, Bernstein BH. Evaluation and management of pain in children with rheumatic diseases. Rheum Dis Clin North Am 1991;17:985–1000. 7. Murray KJ, Passo MH. Functional measures in children with rheumatic diseases. Pediatr Clin North Am 1995;42:1127–53. 8. Varni JW, Wilcox KT, Hanson V, Brik R. Chronic musculoskeletal
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