OBJECTIVE: Measure modification can impact comparability of scores across groups and settings. Changes in items can affect the percent admitting to a symptom. METHODS: Using item response theory (IRT) methods, well-calibrated items can be used interchangeably, and the exact same item does not have to be administered to each respondent, theoretically permitting wider latitude in terms of modification. RESULTS: Recommendations regarding modifications vary, depending on the use of the measure. In the context of research, adjustments can be made at the analytic level by freeing and fixing parameters based on findings of differential item functioning (DIF). The consequences of DIF for clinical decision making depend on whether or not the patient's performance level approaches the scale decision cutpoint. High-stakes testing may require item removal or separate calibrations to ensure accurate assessment. DISCUSSION: Guidelines for modification based on DIF analyses and illustrations of the impact of adjustments are presented.
"The presence of statistically significant item bias leads earlier researchers to recommend dropping items with DIF (e.g., Stommel et al., 1993). As explained in a more recent work (e.g., Teresi et al., 2012), dropping items with DIF is unnecessary. "
"So far, most work has mainly focused on detection of DIF rather than on its actual practical impact . The few studies that have investigated practical impact show mixed results . Some found low practical impact  , whereas others found substantial impact  . "
[Show abstract][Hide abstract] ABSTRACT: Objective
To investigate the impact of differences in depressive symptom reporting across clinical groups (healthcare setting, chronic illness, depression diagnosis and anxiety diagnosis) on clinical interpretability and comparability of depression scores.
Participants from the Netherlands Study of Depression and Anxiety (n = 2981) completed the self-report Inventory of Depressive Symptomatology (IDS-SR). Differences in depressive symptom reporting between distinct clinical subpopulations were assessed using a Differential Item Functioning (DIF) analysis. The effects of DIF on symptom level were evaluated by examining whether DIF-adjustment had clinically relevant effects.
Significant DIF was detected across all tested clinical subpopulation groupings. Clinically relevant DIF was found on the symptom level for 13 IDS-SR items. However, impact of DIF on the aggregate level ranged from small to negligible: adjustment for DIF only led to salient changes in aggregate scores for 0.2-12.7% of individuals across tested sources of DIF.
Differences in endorsement patterns of depressive symptoms were observed across clinical populations, challenging the assumptions regarding the measurement properties of self-reported depression. However, effects of DIF on the aggregate level of IDS-SR total scores were found to be minimal and not clinically important. The IDS-SR thus seems robust against DIF across clinical populations.
Journal of Psychosomatic Research 08/2014; 78(2). DOI:10.1016/j.jpsychores.2014.08.014 · 2.74 Impact Factor
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