The World Health Organization (WHO) Expert Committee on Health Statistics, in its tenth report (Technical Report Series, number 336 of 1966), concluded that in view of the important role played by sampling in many types of public health investigations and the shortage of experts in the theory and practice of sampling, epidemiologists and other health workers should be provided with facilities for obtaining a basic knowledge of sampling principles and methods and acquainted with their potential applications in the medical field. The Committee therefore recommended that a manual dealing with the general principles of sampling and describing in some detail the special problems and opportunities in the medical field would be a useful guide for many workers in public health. The manual would assist the statistician or sampling expert with no previous experience of medical applications, and would also prove valuable for training courses. In 1973 a document: Adequacy of sample size (HSM/73.1) was issued by WHO's Statistical Methodology Unit, as a second edition of a 1961 document (MHO/PA/220.63) with reconstructed tables. Since then the document has been in steady demand. The 1973 document was issued by the then Health Statistical Methodology Unit of WHO in Geneva because "WHO (was) sponsoring a major program in medical research and workers engaged in it needed to have at their elbow a document answering questions on the adequacy of sample size". The current emphasis of the Organization's activities is different to that in 1973. While the tables in HSM/73.1 are still adequate for most purposes of experimental research, they do not cover important areas of case-control type studies and cluster sampling. These approaches are the most likely to be adopted by health managers in evaluating and monitoring their health programs. WHO's Unit of Epidemiological and Statistical Methodology (ESM), in collaboration with the Organization's programs of: Diarrhoeal Disease Control (CDD), Expanded Immunization (EPI) and Research and Training in Tropical Diseases (TDR), sponsored the preparation of this book on the determination of adequate sample sizes under different situations. A number of "typical" questions which health workers pose to the statisticians concerning the size of the sample of subjects they should study are covered in this book. It is hoped that the book will meet the needs of health workers and managers faced with the problem of deciding how large a sample to survey or study, and that it will provide insight into the methodology of solving the most common problems of sample size needs. The authors would like to acknowledge the editorial assistance of James L. Duppenthaler, World Health Organization, in the preparation of this work.

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