"As a cohort of people, animals, or machines ages, the individuals at highest risk tend to die or exit first.  This differential selection can produce patterns of mortality for the population as a whole that are surprisingly different from the patterns for subpopulations or individuals.  Naive acceptance of observed population patterns may lead to erroneous policy recommendations if an intervention depends on the response of individuals.  Furthermore, because patterns at the individual level may be simpler than composite population patterns, both theoretical and empirical research may be unnecessarily complicated by failure to recognize the effects of heterogeneity."