Neglect of dropout underestimates effects of death in longitudinal studies.

Investigations of terminal declines in mental abilities have assessed cognitive performance at a single time point and retrospectively compared survivors and decedents at a single later census date. Neglect of outcomes other than death, such as dropout, causes a loss of information on the relative frailty of survivors and deceased persons before the census date and on incidence of mortality and frailty among survivors after the census date. This discards information on differences in health status between younger and older survivors. The Heim AH4-1 intelligence test was given to 4,228 people between the ages of 42 and 92 years, and both deaths and dropouts were logged during three successive census periods during the subsequent 11 years. Within and across census periods, effects of impending death and dropout did not differ, decreasing with time from initial assessment. Thus the effects of terminal decline, or indeed of any other variable affecting cognitive performance, are miscalculated if dropout is ignored.