This manual presents the practical and statistical information needed to help investigators decide how large a sample to select from a population targeted for a health study or survey. Designed to perform a 'cookbook' function, the book uses explanatory text and abundant tabular calculations to vastly simplify the task of determining the minimum sample size needed to obtain statistically valid results. The objective is to assist those investigators, undertaking health studies at local or district level, who lack detailed knowledge of statistical methodology." French and Spanish editions are in preparation.