This article presents results concerning the significance of sample leads and illustrates their uses. They enable the analysis of one party over another in political polls, one product over another in market research surveys, market share in industrial studies, etc. They enable the calculation of confidence intervals and required sample size, testing of hypotheses, etc. Sampling from infinite and finite populations with overlapping classifications, stratified sampling, optimum allocation to strata, etc., are considered. Derivation of the results is given in a set of Appendices. Some of these are rather complicated, but their uses are straightforward.