Disparate time spans in sequential studies of aging

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Abstract
In several published studies of intelligence and adult age, a sequential design was used in which birth cohort and time of measurement were the independent variables in an ANOVA. The time spans of these two variables were quite disparate; the cohort variable covered about 50 years whereas the time-of-measurement variable covered 7 or 14 years. It was demonstrated here that when time spans are disparate, even when all differences are attributable entirely to age changes, the F values typically will be larger for the cohort variable and errors in interpretations become likely. When cohort differences are statistically significant and time-of-measurement differences are not, the conclusion that age differences are “generational” and not due to age changes within individuals may be wrong.