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All epidemiologic studies benefit from sample size and power calculations in the pilot phase of the research. This graph shows that the sample size needed for a cohort study depends upon, among other things, the percent of the cohort having the outcome of interest. Injury studies with people who do heavy manual materials handling may need a smaller number of subjects than a study of office workers. Where the outcome is frequent, such as 30% of an elderly cohort falling over a six-month period, cohort studies become easier and less expensive to conduct.