Purpose: The Millennium Cohort Study began in 2001 using mail and Internet questionnaires to gather occupational and environmental exposure, behavioral risk factor, and health outcome data from a large, population-based US military cohort. Standardized instruments, including the Patient Health Questionnaire, the Medical Outcomes Study Short Form-36 for Veterans, and the Posttraumatic Stress Disorder (PTSD) Checklist-Civilian Version, have been validated in various populations. The purpose of this study was to investigate internal consistency of standardized instruments and concordance of responses in a test-retest setting.
Methods: Cronbach alpha coefficients were used to investigate the internal consistency of standardized instruments among 76,742 participants. Kappa statistics were calculated to measure stability of aggregated responses in a subgroup of 470 participants who voluntarily submitted an additional survey within 6 months of their original submission.
Results: High internal consistency was found for 14 of 16 health components, with lower internal consistency found among two alcohol components. Substantial test-retest stability was observed for stationary variables, while moderate stability was found for more dynamic variables that measured conditions with low prevalence.
Conclusions: These results substantiate internal consistency and stability of several standard health instruments applied to this large cohort. Such reliability analyses are vital to the integrity of long-term outcome studies.