ABSTRACT
References .............................................................................................................................................. 485 Exercises ............................................................................................................................. 485
When free-living people are members of a cohort under long-term observation, a baseline “zerotime” status can be identified for each person, but special problems and challenges can arise thereafter. The final outcome may be unknown for members of the cohort who “drop out” or become “lost to
follow-up.” The intermediate events may be unknown or difficult to interpret for persons who continue to participate in the study, but who make unauthorized changes in the assigned plans or fail to appear for scheduled examinations.
