Analysis of data with excess zeros.

This paper is an introduction to a set of papers on two-part models that were presented at the 2000 Joint Statistical Meetings of American Statistical Association, International Biometric Society (ENAR), International Biometric Society (WNAR), Institute of Mathematical Statistics, and Statistical Society of Canada. The second part of the paper summarizes results of a study of the size and power of two-part models. It proposes that two-part models are a useful alternative to the t-test and the Wilcoxon test.