Effects of sample attrition in a longitudinal study of the association between alcohol intake and all-cause mortality

Longitudinal studies show higher mortality among abstainers and heavy drinkers than among light and moderate alcohol consumers. The influence on this association of missing information on alcohol intake due to attrition (dropout) has not been examined previously. The aims of this study were to characterize participants who dropped out and to evaluate whether the missing information influenced the association between alcohol intake and all-cause mortality.

Data on the 18 974 participants in the Copenhagen City Heart Study, with four measures of alcohol intake and other life-style factors during 28 years of follow-up, were linked with nation-wide registers on socio-economic covariates, mortality and disease incidence. Logistic regression was used to describe life-style and socio-economic determinants of attrition, and Poisson regression was used to evaluate how attrition affected the association between alcohol intake and mortality. The statistical methods used for dealing with missing values were complete case analysis, carry last observation forward, simple imputations, multiple imputation and weighting.

Abstinence and high alcohol intake, current smoking, physical inactivity and high body mass index increased the odds of dropping out, whereas being married, more years of education, skilled occupation, high income and large residential area decreased the odds. Attrition was associated with increased mortality and incidence rates of heart disease, lung and upper digestive tract cancers and alcoholic liver diseases. Increased mortality among abstainers and heavy drinkers was observed with all methods used for handling missing data on alcohol intake.

Attrition was non-random, and the observed association between alcohol intake and all-cause mortality did not differ by statistical method for handling missing data.