Bias related to the exclusion of the economically inactive in studies on social class differences in mortality

BACKGROUND: To assess how the exclusion of the economically inactive affects levels and trends in social class differences in mortality among men and women at different durations of follow-up. METHODS: Records of the 1970, 1975, 1980 and 1985 censuses on Finnish men and women aged 35-64 linked with records of all deaths during 1971-1990. RESULTS: Exclusion of the economically inactive population underestimates the class differences in the total population by about 25% among men and 60% among women. The bias does not disappear if the first 5 years of follow-up are excluded and the bias can lead to erroneous conclusions about the trends in social class differences in mortality. CONCLUSIONS: Analyses based on the economically active population may lead to significant underestimation of social class differences in mortality, introduce biases in international comparison and may only partially capture the causal mechanisms underlying these mortality differences. Our results further show that although the bias diminishes during the follow-up, it is by no means eliminated after the first 5 years. The underestimation of social class differences in mortality created by the exclusion of the inactive should be more widely recognized and more accurate data on previous occupations should be collected.