One limitation of this study is small sample size. It is possible that a larger sample may have provided the power necessary to replicate the previously demonstrated associations between smoking and other chronic diseases examined in this study. Other limitations derive from the homogeneity of the sample and have been outlined previously (Vaillant, 1979, 1998). The sample is educationally and racially homogeneous and was pre-selected for good health. However, the selection of these men for their health minimizes the possibility of a pre-existing condition, thus maximizing the probability that the conditions examined were developed during the observation period. Moreover, while the homogeneity of the sample limits the generalizability of the results, it eliminates the need to control for factors such as education, socio-economic class, race, and gender, which would dramatically increase sample size requirements.
