Long before I had any inkling that I would write a book, I learned that apreface is the authors attempt to engage the reader in a leisurely conversation before serious joint work begins. In that spirit, I ask you to spend a moment with me now. Although an important focus of my training and daily work is in mathematics and statistics, I have found that many health care workers in general, and physicians in particular, have difficulty understanding research issues when they are presented mathe­ matically. Statistical principles in medicine are relatively straight­ forward and can be easily absorbed without a heavy mathematical preamble. I have come to believe that the underlying research principles are not difficult; what is difficult is the mathematics in which the principles are embedded. The mathematical medium often distorts and confuses the research message for nonmathe­ maticians. I wrote this book to explain the statistical principles in health care in fairly nonmathematical terms.