Assessment of depression in veterans across missions: a validity study using Rasch measurement models

Background: Depression is a common psychopathological outcome following military deployment. Previous studies have reported differing rates of post-deployment depression, indicating that the toll of war differs across missions. However, it is unclear to what degree the varying prevalence is due methodological differences. Studies comparing rates of depression across cohorts using the same methodology and ensuring measurement invariance are rare, leaving us with limited knowledge on the actual depression prevalence variance across missions. Objective: Applying Rasch models (RM), we aim to validate a measure of depression distributed to all personnel deployed with the Danish Defense since 1998. The main focus was establishing a sufficient sum score and measurement invariance relative to deployment cohort. Method: Two cohorts of the International Security Assistance Force (ISAF) deployed to Afghanistan in 2009 (ISAF7, N = 265) and 2013 (ISAF15, N = 271) were included. Participants filled out a questionnaire concerning their Psychological Reactions to International Missions (PRIM) approximately seven months after home-coming. The questionnaire included a 10-item scale of depression symptoms (PRIM-Depression). The validity of the PRIM-Depression was tested using RM with specific focus on differential item functioning (DIF) across the two cohorts. Results: The PRIM-Depression scale displayed excellent overall consistency and showed no problems with monotonicity or homogeneity. However, the full PRIM-Depression scale did not fit a pure RM. We therefore tested the fit of items to a graphical log-linear RM and found evidence of DIF for two items relative to cohort. We proceeded without these two items and tested the resulting 8-item version which fitted a pure RM without DIF on any of the exogenous variables. Conclusions: Our results suggest that the 10-item PRIM-Depression scale should be used to compare cohorts only with appropriate score equation. The 8-item version provides a sufficient statistic and can as such be applied using the raw score.