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Many capitalist economies are characterised by business cycles with concomitant increases in joblessness during recessions and depressions, and reductions in unemployment in periods of economic expansion. In view of the potentially debilitating consequences of joblessness on health and related outcomes, research on unemployment and suicide continues to be relevant in both epidemiology and sociology. One controversy that continues is the issue of selection bias. The essential question that remains unresolved is whether the observed association between unemployment and suicide reported in some studies reflects direct causation or whether there is some variable that is causally prior to both unemployment and suicide.
The report by Blakely et al 1 presents an analysis of the New Zealand Census Mortality Study (NZCMS) that attempts to shed some light on the above question. Using logistic regression models on census mortality linked data on 1.65 million men and women aged 18 to 64 years, they have observed that unemployment is strongly associated with suicide among women and men in the age group 25-64. At the same time, no significant associations were observed in other age groups. In an effort to support a causal argument, the authors have controlled for the usual socioeconomic variables (education and income), and to convince readers that there is no confounding (selection bias) Blakely et al 1 have also reported results of various sensitivity analyses using information from other studies.
The analysis was competently done, but the study is not without serious limitations. Firstly, the key independent variable in the report, employment status is a time varying covariate, but it is not treated as such in the analysis. Failure to account for...