Health, medical, and social scientists collect and analyze data about individuals as a regular part of their research and clinical activities. Some of these data indicate the presence of diseases or other phenom- ena of interest. Other variables are attributes thought to be associated with the responses of interest. Finally, individuals have characteristics that may affect the associations being studied. When the variable being studied or the "response" takes on a limited number of values, the method of analysis requires the techniques that are the subject of this book.