Cohort studies have particular advantages in confirming results of retrospective or case-control studies in those situations in which case-control studies are no longer feasible. In circumstances, the cohort study may involve randomization, thus reducing selection bias, but ordinarily there will have been self-selection by individuals as to the group in which they will fall. Investigators should analyze data from a cohort study so as to take the passage of time into account. Variables anticipated to have effects should be accounted for by stratification, if feasible, or by mathematical modeling, if necessary. Results should be interpreted with care, and qualifications should be made on any interpretations, including qualifications relating to the propriety of the mathematical model used. When long latencies are a factor, and particularly when exposure is initiated late in life, establishment of a positive role for the exposure can be difficult. Case-control and other epidemiologic studies are biased toward identification of exposures leading to outcomes of a unique nature but fail to identify more serious exposures with adverse outcomes which are more commonplace.