Abstract

BackgroundDifferential Item Functioning (DIF) is investigated to ensure that each item displays a consistent pattern of responses irrespective of the characteristics of the respondents. Assessing DIF helps to understand the nature of instruments, to assess the quality of a measure and to interpret results. This study aimed to examine whether the items of the Quality of Life Enjoyment and Satisfaction Questionnaire-Short Form (Q-LES-Q-SF) and Short-Form 12 (SF-12) exhibit DIF.MethodA total of 124 outpatients diagnosed with substance dependence participated in a cross-sectional, multicenter study. In addition to the Q-LES-Q-SF and SF-12 results, demographic data such as age, sex, type of substance dependence and education level were collected. Rasch analysis was conducted (using RUMM2020 software) to assess DIF of the Q-LES-Q-SF and SF-12 items.ResultsFor SF-12, significant age-related uniform DIF was found in two of the 12 items, and sex-related DIF was found in one of the 12 items. All of the observed DIF effects in SF-12 were found among the mental health items. Three items showed DIF on the Q-LES-Q-SF; however, the impact of DIF item on the delta score calculation for the comparisons of self-reported health status between the groups was minimal in the SF-12 and small in the Q-LES-Q-SF.ConclusionThese results indicated that no major measurement bias affects the validity of the self-reported health status assessed using the Q-LES-Q-SF or SF-12. Thus, these questionnaires are largely robust measures of self-reported health status among substance users.

Highlights

  • Differential Item Functioning (DIF) is investigated to ensure that each item displays a consistent pattern of responses irrespective of the characteristics of the respondents

  • Three items showed DIF on the Q-LES-Q-SF; the impact of DIF item on the delta score calculation for the comparisons of self-reported health status between the groups was minimal in the Short-Form 12 (SF-12) and small in the Q-LES-Q-SF

  • These results indicated that no major measurement bias affects the validity of the self-reported health status assessed using the Q-LES-Q-SF or SF-12

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Summary

Introduction

Differential Item Functioning (DIF) is investigated to ensure that each item displays a consistent pattern of responses irrespective of the characteristics of the respondents. Assessing DIF helps to understand the nature of instruments, to assess the quality of a measure and to interpret results. Generic and specific instruments are applied to measure the self-reported health status of substance. Bourion-Bédès et al Health and Quality of Life Outcomes (2015) 13:172 instrument because a key assumption in measurement is that characteristics of the respondents that are unrelated to the construct being measured (e.g., country, language or culture of the respondents) do not affect their responses to the items [5, 6]. DIF can result in biased between-group comparisons because the response patterns may reflect attributes other than that which the instrument is intended to measure [10,11]

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