The third edition of this successful textbook is a lucid introduction to the principles and elementary techniques of statistical reasoning. New materials are included that show the usefulness of computers to biologists; computer analyses of a selection of the examples are presented, using SPSS-X, Minitab and Genstat. The book assumes no mathematical training and uses a minimum of jargon and symbolism. Particular attention is paid to the validity and use of statistical procedures, the interpretation of results, and the meanings of conclusions. Full explanations of statistical methods are given.