Exploring differential item functioning in the SF-36 by demographic, clinical, psychological and social factors in an osteoarthritis population

Beth Pollard*, Marie Johnston, Diane Dixon

*Corresponding author for this work

Research output: Contribution to journalArticle

9 Citations (Scopus)
3 Downloads (Pure)

Abstract

Background
The SF-36 is a very commonly used generic measure of health outcome in osteoarthritis (OA). An important, but frequently overlooked, aspect of validating health outcome measures is to establish if items work in the same way across subgroup of a population. That is, if respondents have the same ‘true’ level of outcome, does the item give the same score in different subgroups or is it biased towards one subgroup or another. Differential item functioning (DIF) can identify items that may be biased for one group or another and has been applied to measuring patient reported outcomes. Items may show DIF for different conditions and between cultures, however the SF-36 has not been specifically examined in an osteoarthritis population nor in a UK population. Hence, the aim of the study was to apply the DIF method to the SF-36 for a UK OA population.

Methods
The sample comprised a community sample of 763 people with OA who participated in the Somerset and Avon Survey of Health. The SF-36 was explored for DIF with respect to demographic, social, clinical and psychological factors. Well developed ordinal regression models were used to identify DIF items.

Results
DIF items were found by age (6 items), employment status (6 items), social class (2 items), mood (2 items), hip v knee (2 items), social deprivation (1 item) and body mass index (1 item). Although the impact of the DIF items rarely had a significant effect on the conclusions of group comparisons, in most cases there was a significant change in effect size.

Conclusions
Overall, the SF-36 performed well with only a small number of DIF items identified, a reassuring finding in view of the frequent use of the SF-36 in OA. Nevertheless, where DIF items were identified it would be advisable to analyse data taking account of DIF items, especially when age effects are the focus of interest.
Original languageEnglish
Article number346
JournalBMC Musculoskeletal Disorders
Volume14
DOIs
Publication statusPublished - 11 Dec 2013

Keywords

  • osteoarthritis
  • SF-36
  • psychometrics
  • item bias
  • differential item functioning
  • measurement equivalence
  • quality-of-life
  • health-assessment questionnaire
  • knee replacment surgery
  • logistic-regression
  • Rasch analysis
  • rheumatoid-arthritis
  • physical function
  • validity
  • scales
  • issues

Cite this

@article{6c465e138dbe4c77b646e97c18311ae1,
title = "Exploring differential item functioning in the SF-36 by demographic, clinical, psychological and social factors in an osteoarthritis population",
abstract = "BackgroundThe SF-36 is a very commonly used generic measure of health outcome in osteoarthritis (OA). An important, but frequently overlooked, aspect of validating health outcome measures is to establish if items work in the same way across subgroup of a population. That is, if respondents have the same ‘true’ level of outcome, does the item give the same score in different subgroups or is it biased towards one subgroup or another. Differential item functioning (DIF) can identify items that may be biased for one group or another and has been applied to measuring patient reported outcomes. Items may show DIF for different conditions and between cultures, however the SF-36 has not been specifically examined in an osteoarthritis population nor in a UK population. Hence, the aim of the study was to apply the DIF method to the SF-36 for a UK OA population.MethodsThe sample comprised a community sample of 763 people with OA who participated in the Somerset and Avon Survey of Health. The SF-36 was explored for DIF with respect to demographic, social, clinical and psychological factors. Well developed ordinal regression models were used to identify DIF items.ResultsDIF items were found by age (6 items), employment status (6 items), social class (2 items), mood (2 items), hip v knee (2 items), social deprivation (1 item) and body mass index (1 item). Although the impact of the DIF items rarely had a significant effect on the conclusions of group comparisons, in most cases there was a significant change in effect size.ConclusionsOverall, the SF-36 performed well with only a small number of DIF items identified, a reassuring finding in view of the frequent use of the SF-36 in OA. Nevertheless, where DIF items were identified it would be advisable to analyse data taking account of DIF items, especially when age effects are the focus of interest.",
keywords = "osteoarthritis, SF-36, psychometrics, item bias, differential item functioning, measurement equivalence, quality-of-life, health-assessment questionnaire, knee replacment surgery, logistic-regression, Rasch analysis, rheumatoid-arthritis, physical function, validity, scales, issues",
author = "Beth Pollard and Marie Johnston and Diane Dixon",
year = "2013",
month = "12",
day = "11",
doi = "10.1186/1471-2474-14-346",
language = "English",
volume = "14",
journal = "BMC Musculoskeletal Disorders",
issn = "1471-2474",
publisher = "BMC",

}

TY - JOUR

T1 - Exploring differential item functioning in the SF-36 by demographic, clinical, psychological and social factors in an osteoarthritis population

AU - Pollard, Beth

AU - Johnston, Marie

AU - Dixon, Diane

PY - 2013/12/11

Y1 - 2013/12/11

N2 - BackgroundThe SF-36 is a very commonly used generic measure of health outcome in osteoarthritis (OA). An important, but frequently overlooked, aspect of validating health outcome measures is to establish if items work in the same way across subgroup of a population. That is, if respondents have the same ‘true’ level of outcome, does the item give the same score in different subgroups or is it biased towards one subgroup or another. Differential item functioning (DIF) can identify items that may be biased for one group or another and has been applied to measuring patient reported outcomes. Items may show DIF for different conditions and between cultures, however the SF-36 has not been specifically examined in an osteoarthritis population nor in a UK population. Hence, the aim of the study was to apply the DIF method to the SF-36 for a UK OA population.MethodsThe sample comprised a community sample of 763 people with OA who participated in the Somerset and Avon Survey of Health. The SF-36 was explored for DIF with respect to demographic, social, clinical and psychological factors. Well developed ordinal regression models were used to identify DIF items.ResultsDIF items were found by age (6 items), employment status (6 items), social class (2 items), mood (2 items), hip v knee (2 items), social deprivation (1 item) and body mass index (1 item). Although the impact of the DIF items rarely had a significant effect on the conclusions of group comparisons, in most cases there was a significant change in effect size.ConclusionsOverall, the SF-36 performed well with only a small number of DIF items identified, a reassuring finding in view of the frequent use of the SF-36 in OA. Nevertheless, where DIF items were identified it would be advisable to analyse data taking account of DIF items, especially when age effects are the focus of interest.

AB - BackgroundThe SF-36 is a very commonly used generic measure of health outcome in osteoarthritis (OA). An important, but frequently overlooked, aspect of validating health outcome measures is to establish if items work in the same way across subgroup of a population. That is, if respondents have the same ‘true’ level of outcome, does the item give the same score in different subgroups or is it biased towards one subgroup or another. Differential item functioning (DIF) can identify items that may be biased for one group or another and has been applied to measuring patient reported outcomes. Items may show DIF for different conditions and between cultures, however the SF-36 has not been specifically examined in an osteoarthritis population nor in a UK population. Hence, the aim of the study was to apply the DIF method to the SF-36 for a UK OA population.MethodsThe sample comprised a community sample of 763 people with OA who participated in the Somerset and Avon Survey of Health. The SF-36 was explored for DIF with respect to demographic, social, clinical and psychological factors. Well developed ordinal regression models were used to identify DIF items.ResultsDIF items were found by age (6 items), employment status (6 items), social class (2 items), mood (2 items), hip v knee (2 items), social deprivation (1 item) and body mass index (1 item). Although the impact of the DIF items rarely had a significant effect on the conclusions of group comparisons, in most cases there was a significant change in effect size.ConclusionsOverall, the SF-36 performed well with only a small number of DIF items identified, a reassuring finding in view of the frequent use of the SF-36 in OA. Nevertheless, where DIF items were identified it would be advisable to analyse data taking account of DIF items, especially when age effects are the focus of interest.

KW - osteoarthritis

KW - SF-36

KW - psychometrics

KW - item bias

KW - differential item functioning

KW - measurement equivalence

KW - quality-of-life

KW - health-assessment questionnaire

KW - knee replacment surgery

KW - logistic-regression

KW - Rasch analysis

KW - rheumatoid-arthritis

KW - physical function

KW - validity

KW - scales

KW - issues

U2 - 10.1186/1471-2474-14-346

DO - 10.1186/1471-2474-14-346

M3 - Article

VL - 14

JO - BMC Musculoskeletal Disorders

JF - BMC Musculoskeletal Disorders

SN - 1471-2474

M1 - 346

ER -