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Both cross-sectional and longitudinal designs are limited in that they confound age, cohort, and period effects. When sequential designs—which only examine two such constructs at a time—are utilized to disentangle these effects, important findings emerge. For example, many cross-sectional and longitudinal studies have found that alcohol consumption decreases with age, but they have utilized only a few cohorts. When analyzing across cohorts, strong period effects emerge, such that levels of drinking increased regardless of age and cohort between 1973-1982, whereas they decreased between 1982-1991 for all ages and cohorts (perhaps partially explained by public health campaigns designed to reduce consumption).

A second study examined changes in values over time for several groups of college (UC Davis) graduates. When just considering one cohort in a longitudinal design, it appeared that “family orientation” decreased during the twenties, and then increased during the thirties. However, when examining several cohorts in sequential designs, it was found that between 1979 and 1991, all subjects (regardless of age and cohort) increased in family orientation.

There were differences in Aldwyn’s studies between WWII and Vietnam in levels
of PTSD symptoms, but they could not distinguish between cohort and aging
effects in that study.

There is a crucial need for more sequential designs in longitudinal research in order to disentangle age, period, and cohort effects and allow examination of more refined questions concerning the aging process.

Three key themes that emerged from this set of talks and discussion: (Phil Cowan)
1. Consideration of how best to measure change over time.
2. The importance of sound theory to underlie all hypotheses and expectations, both what we expect and what we do not expect.
3. Whether and how studies can be generalized from given that each is unique, time-consuming, and expensive.

Carolyn Aldwin's presentation "Disentangling Age, Cohort, and Period Effects" can be viewed in PDF format, using Adobe® Acrobat® Reader®.