Cohort effects: A possible limitation to the interpretation of longitudinal studies

The possible limitation that cohort effects could impose upon the interpretation of longitudinal research has been examined, taking data on the standing height and the maximal oxygen intake of 546 primary school students participating in the Trois Rivières mixed‐longitudinal study of enhanced physical education programs (295 experimental and 251 control subjects of both sexes). Over the short period of the study (maximum intercohort time span of 4 years), no significant differences in standing height were observed. In the larger urban community of Trois Rivières (population about 100,000), where the program of additional physical education was very successful in augmenting the maximal oxygen intake of the experimental students, a statistically significant intercohort difference of aerobic power developed among control students over the span of 3–4 years. However, in Pont Rouge (population about 5,000), where the added physical education had a smaller impact upon the maximal oxygen intake of the experimental students, there were no significant intercohort differences. Among possible explanations of the intercohort difference seen in Trois Rivières, the most likely seems an indirect effect of the experimental physical education program upon the patterns of habitual physical activity in the control population. © 1993 Wiley‐Liss, Inc.