EXAMINATION OF EARLY MORTALITY EXCLUSION AS AN APPROACH TO CONTROL FOR CONFOUNDING BY OCCULT DISEASE IN EPIDEMIOLOGIC STUDIES OF MORTALITY RISK-FACTORS

Methods for the estimation of the effects of chronic disease risk fact ors on mortality continue to be an area that generates confusion and c ontroversy, In response to the frequently observed U- or J-shaped rela tions between risk factors and mortality, some authors suggest that su bjects dying during the first k years of follow-up (where k is some po sitive number less than the total length of follow-up) be excluded fro m statistical analyses, By excluded, the authors mean completely remov ed from the data set. The rationale is that persons dying during the f irst k years are likely to have a preexisting occult disease that conf ounds the relation between the risk factor under study and mortality, Excluding persons dying during the first k years of follow-up purporte dly reduces this confounding. However, the authors are aware of no dem onstration that this procedure effectively accomplishes its goal, They shaw that excluding subjects who die during the first k years of foll ow-up does not necessarily lead to a reduction in bias in the estimate d effect of a risk factor on mortality when this relation is confounde d by the presence of occult disease, Moreover, it is possible for such exclusion to exacerbate the confounding due to preexisting disease, T hus, excluding subjects dying during the first k years of follow-up is not necessarily an effective strategy for dealing with confounding du e to occult disease, Investigators are encouraged to pursue alternativ e methods.