This book will give ophthalmologists a simple, clear presentation of epidemiologic and statistical techniques relevant to conducting, inter­ preting, and assimilating the commonest types of clinical research. Such information has become all the more important as clinical ophthalmic research has shifted from anecdotal reports and un­ critical presentations of patient data to carefully controlled and organized studies.

The text is divided into two parts. The first part deals with epi­ demiologic concepts and their application and with the various types of studies (for example, prospective or longitudinal) and their organization (use of controls, randomness, sources of bias, choice of sample size, standardization and reproducibility). The second half of the text is devoted to the selection and use of the relevant sta­ tistical manipulations: determining sample size and testing the sta­ tistical significance of results. The text is illustrated with many examples covering topics such as blindness registry data, incidence of visual field loss with different levels of ocular hypertension, sensi­ tivity and specificity of tests for glaucoma, and sampling bias in studies of the safety of intraocular lenses. References and several helpful appendices are included.

The Author

Alfred Sommer received his M.D. in Ophthalmology from Harvard University Medical School, and his M.H. Sc. degree in Epidemiology from Johns Hopkins School of Hygiene and Public Health. At the Wilmer Institute, he is the Director of the International Center for Epidemiologic and Preventive Ophthalmology. He is the author of

Field Guide to the Detection and Control of Xerophthalmia.

Oxford University Press, New York

Epidemiology

and Statistics

for the

Ophthalmologist

ALFRED SOMMER, M.D., M.R.Se.

The Wilmer Ophthalmological Institute

of Johns Hopkins University,

and Helen Keller International

New York Oxford

OXFORD UNIVERSITY PRESS

1980

Copyright © 1980 by Oxford University Press, Inc.

LIBRARY OF CONGRESS IN PUBLICATION DATA

Sommer, Alfred, 1942Epidemiology and statistics for the

ophthalmologist.

Bibliography: p.

Includes index.

1.Ophthalmology-Statistical methods.

2.Epidemiology-Statistical methods. I. Title. [DNLM: 1. Epidemiologic methods. 2. Statistics. WA950 S697e1

Printed in the United States of America

Preface

There are three kinds of lies: lies, damned lies, and statistics.

BENJAMIN DISRAELI

Until recently, epidemiologic and statistical principles were rou­ tinely ignored in ophthalmic research. Fortunately thisisnolonger the case, since their use results in better designed, more effi­ cient, and meaningful studies-usually with little additional work. It is essential that every clinical investigator be familiar with the concepts involved: where outside epidemiologic and statistical assistance is available, he must be able to recognize what help to seek and questions to ask; and where not available, he must be able to carry out the necessary procedures on his own. It is equally essential for every informed clinician who wishes to be better equipped to evaluate the significance and value of pub­ lished-and sometimes conflicting-reports.

This is a practical primer for busy clinicians who have neither the time nor inclination to pursue formal courses in epidemiology and statistics. For simplicity and brevity we confine ourselves to principles and techniques required for conducting and interpret­ ing the most common types of clinical studies: descriptive reports seeking new etiologic agents, evaluations of diagnostic and screening procedures, and therapeutic trials.

I can only hope my colleagues will ultimately disagree with Disraeli, and find the subject interesting and above all, useful.

September 1979