Concepts of Epidemiology: An integrated introduction to the ideas, theories, principles and methods of epidemiology Raj S. Bhopal OXFORD UNIVERSITY PRESS bhop-fm 3/8/02 2:16 PM Page i Concepts of Epidemiology An integrated introduction to the ideas, theories, principles and methods of epidemiology This page intentionally left blank Concepts of Epidemiology An integrated introduction to the ideas, theories, principles and methods of epidemiology Raj S. Bhopal Alexander Bruce and John Usher Professor of Public Health and Head, Department of Community Health Sciences University of Edinburgh, Scotland formerly Professor of Epidemiology and Public Health University of Newcastle Upon Tyne 1 bhop-fm 3/8/02 2:16 PM Page iv 1 Great Clarendon Street, Oxford OX2 6DP Oxford University Press is a department of the University of Oxford. It furthers the University’s objective of excellence in research, scholarship, and education by publishing worldwide in Oxford New York Auckland Bangkok Buenos Aires Cape Town Chennai Dar es Salaam Delhi Hong Kong Istanbul Karachi Kolkata Kuala Lumpur Madrid Melbourne Mexico City Mumbai Nairobi São Paulo Shanghai Taipei Tokyo Toronto and an associated company in Berlin Oxford is a registered trade mark of Oxford University Press in the UK and in certain other countries Published in the United States by Oxford University Press Inc., New York © R. S. Bhopal, 2002 The moral rights of the author have been asserted Database right Oxford University Press (maker) First published 2002 All rights reserved. No part of this publication may be reproduced, stored in a retrieval system, or transmitted, in any form or by any means, without the prior permission in writing of Oxford University Press, or as expressly permitted by law, or under terms agreed with the appropriate reprographics rights organization. Enquiries concerning reproduction outside the scope of the above should be sent to the Rights Department, Oxford University Press, at the address above You must not circulate this book in any other binding or cover and you must impose this same condition on any acquirer British Library Cataloguing in Publication Data Data available Library of Congress Cataloging in Publication Data Bhopal, Raj S. Concepts of epidemiology : an integrated introduction to the ideas, theories, principles, and methods of epidemiology / by Raj S. Bhopal. Includes bibliographical references. 1. Epidemiology. I. Title. [DNLM: 1. Epidemiology. WA 105 B575c 2002] RA650 .B48 2002 614.4—dc21 2001052327 ISBN 0 19 263155 1 (Pbk.) 10987654321 Typeset in 10\12 Minion by Newgen Imaging Systems (P) Ltd, Chennai, India. Printed in Great Britain on acid-free paper by Biddles Ltd, Guildford & King’s Lynn bhop-fm 3/8/02 2:16 PM Page v Foreword When I was learning epidemiology in the late 1950s, I was inspired by the first edition of Jerry Morris’s now classic little monograph, Uses of Epidemiology, but there were hardly any comprehensive current textbooks to guide me. The few available books were either unreadable, unhelpful, or failed to orient my thoughts along the directions my ideas were taking. How times have changed! Now there are so many good books that it is difficult for the uninitiated to select the most suitable one to meet their needs. In 1997, Raj Bhopal reviewed twenty-five textbooks of epidemiology, discussing their approach to the subject, and their strengths and weaknesses, in a critical commentary that is helpful to teachers and learners alike. Now, from the University of Edinburgh (where I spent five happy years in the 1960s), Raj Bhopal, who holds the Usher chair of public health, has written his own introductory textbook for graduate students who are embarking upon the detailed study of epidemiology. This is an excellent introduction. Raj Bhopal’s approach is conceptual—he describes and explains the underlying concepts and methods of epidemiology with clarity and with apt examples, and simple, elegant illustrations. Frequently throughout the text he asks penetrating questions that will test the limits of his readers’ intellectual capacity— an admirable feature that other authors could copy with benefit to themselves and their readers. All the essentials are here: the person-population dyad, variation, error, bias, confounding, causality, the spectrum of disease, the ‘iceberg’ concept, risk and its relationship to disease frequency, study design, the ethical framework within which we practise epidemiology and conduct research, the relationship of epidemiology to other scholarly pursuits, and finally, some thoughts about the way the discipline has evolved and is likely to continue to evolve in the lifetime of those now entering upon careers in this field. If I may speak directly to students starting the study of epidemiology: you can be grateful that there is a book like this to guide you along the fascinating pathway that leads to epidemiological enlightenment and understanding. This book will enable you to comprehend the connections between individual and population health, the natural history of disease, the methods of epidemiology, the interventions that work and don’t work, and the role of epidemiology as the fundamental public health science. This is a book for you to buy, to read, to study, and to enjoy. John M. Last Emeritus Professor of Epidemiology, University of Ottawa bhop-fm 3/8/02 2:16 PM Page vi I dedicate this book to my mother Bhagwanti Kaur Bhopal for impressing on me (and my siblings) the importance of education, and encouraging us to make up for her own lack of schooling and formal education; and to my father Jhanda Singh Bhopal for setting an example of how to work hard, shoulder responsibility, and strive for self-improvement. bhop-fm 3/8/02 2:16 PM Page vii Preface The purpose of this book is to explain and illustrate the key concepts which underpin the science of epidemiology and its applications to research, policy making, health service planning and health promotion. The book emphasizes theory, ideas, and epidemiological axioms. In doing this I hope to counter the mounting criticism that epidemiology is an atheoretical discipline. A concept is an idea, but the word is usually reserved for complex, or interrelated, ideas. A concept is the idea behind the word or phrase we use to describe something. This book, then, aims to explain the ideas underlying the language, principles, and basic methods in epidemiology. For example, the attributable risk and odds ratios are not considered merely as arithmetical equations or tools, but also in terms of the ideas underlying their calculation, applications, strengths, and limitations. This book is primarily written for postgraduates beginning courses on epidemiology anywhere in the world, for the concepts are the same everywhere. Only the examples will differ. The book may be of interest to public health and other epidemiological practi- tioners interested in revisiting the fundamental ideas of their discipline. Undergraduates who are keen on epidemiology may find it helpful in deepening their understanding generally, or while studying some topics in a little depth. Health professionals (including busy doctors) may find this material an interesting adjunct to their use of epidemiologi- cal techniques or data. Finally, health service managers and policy makers may find this book a source of insights into the world of epidemiology. There are 10 chapters. Many introductory courses are designed around 10–15 or so sessions. I envisage that the core of this book could be grasped in 10 days of committed study, preferably in the context of a taught course, but also independently. The book is written in plain language but a basic understanding of biology is needed, as is some familiarity with illness and disease. However, medical terminology is explained and defined in a glossary. The learning objectives are expressed in terms of the reader acquiring understanding. I believe that achieving understanding is the high- est form of learning; from that may flow a lasting and usable knowledge base, change of attitude, and the achievement of skills. There are exercises to help readers to deepen their understanding. Each chapter ends in a summary. The motivation to write this book came from two directions, one in academia, the other in public health practice. As an examiner of both undergraduate and postgradu- ate students I was surprised, and crestfallen, to see how many students could not clearly explain basic ideas such as the difference between a case-control and cohort study, sometimes even after a year of study. In my duties in the health service I partici- pated in many discussions on why service demands exceeded supply even after new bhop-fm 3/8/02 2:16 PM Page viii viii PREFACE investments were made. Not once did anyone invoke the crucial concept of the iceberg of disease and symptoms to explain this phenomenon, and clearly even those who knew of it did not make the leap from the classroom to the boardroom. This book is deliberately discursive, not simply descriptive, to help readers to achieve a deeper understanding and to help bridge the world of theory to that of practice. Students rapidly grasp the importance and worth of studying concepts, and demand more. The depth of understanding of concepts gained from most books and courses is insufficient to permit students to apply what they have learned to the problems they are to solve. The acid test of this book is whether readers find themselves using epi- demiological concepts in their everyday work, not seeing them merely as theoretical constructs for the classroom and examination hall. The conceptual frameworks within which the practice of epidemiology operates take a slightly different perspective from those of the science of epidemiology. The nature of research questions, the relative value of the various methods, and the approach to data analysis, presentation, and interpretation differ. This book demonstrates these differ- ences and makes the implications explicit.