Mitoses from 21 individuals were examined to characterize the within and between subjects distributions of sister chromatid exchange (SCE) frequencies. The importance of accounting for both the within subjects and between subjects variability in the design and interpretation of research is demonstrated. A computer simulation study was performed to evaluate the estimate of an individual's mean SCE rate. A power analysis was performed using our transformed descriptive data to determine the percentage difference in SCE which could be detected between two groups (with 95% probability) as a function of both the number of mitoses per individual and the number of individuals in each group. Tables are provided as guidelines. Conclusions from our work include: (1) The empirical distribution of SCEs within an individual is best described by a negative binomial distribution; (2) The predominant source of variability in baseline SCE rates is between cells within a given individual rather than between individuals and can be accounted for in experiments by counting adequate numbers of mitoses perindividual; and (3) There is a systematic relationship between the mean and SD for SCEs which should be removed prior to hypothesis testing by taking either a logarithmic transformation or square root transformation of the data.