Mental health service use among Canadian veterans within the first 5 years following service: methodological considerations for comparisons with the general population

Introduction: Previous research comparing veteran and civilian mental health (MH) outcomes often assumes stable rates of MH service use over time and relies on standardisation or restriction to adjust for differences in baseline characteristics. We aimed to explore the stability of MH service use in the first 5 years following release from the Canadian Armed Forces and the Royal Canadian Mounted Police, and to demonstrate the impact of using increasingly stringent matching criteria on effect estimates when comparing veterans with civilians, using incident outpatient MH encounters as an example. Methods: We used administrative healthcare data from veterans and civilians residing in Ontario, Canada to create three hard-matched civilian cohorts: (1) age and sex; (2) age, sex and region of residence; and (3) age, sex, region of residence and median neighbourhood income quintile, while excluding civilians with a history of long-term care or rehabilitation stay or receipt of disability/income support payments. Extended Cox models were used to estimate time-dependent HRs. Results: Across all cohorts, time-dependent analyses suggested that veterans had a significantly higher hazard of an outpatient MH encounter within the first 3 years of follow-up than civilians, but differences were attenuated in years 4-5. More stringent matching decreased baseline differences in unmatched variables and shifted the effect estimates, while sex-stratified analyses revealed stronger effects among women compared with men. Conclusions: This methods-focused study demonstrates the implications of several study design decisions that should be considered when conducting comparative veteran and civilian health research.