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.
Methods: 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.
Results: 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.
Conclusion: 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.
Keywords: Depression; Differential item functioning; IDS-SR; Item response theory; Measurement.
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