Essential Epidemiology An Introduction for Students and Health Professionals Second Edition Penny Webb, MA (Cambridge), DPhil (Oxford), is a Senior Research Fellow at the Queensland Institute of Medical Research and Associate Professor in the School of Population Health, University of Queensland. She has worked as a visiting scientist at the International Agency for Research on Cancer, France, and Harvard University, USA and has published more than 120 original research papers in the field of cancer epidemiology. Chris Bain, MB BS (UQ), MPH, MSc (Harvard) is Reader in Epidemiology in the School of Population Health, University of Queensland. He has been teaching epidemiology to public health and medical students for over 3 decades and has co-authored a book on how to conduct a systematic review as well as more than 100 original epidemiology research papers. He has worked at Harvard in the USA and as a visiting researcher at the London School of Hygiene and Tropical Medicine and at the Universities of Cambridge, Oxford and Bristol in the UK.
Essential Epidemiology An Introduction for Students and Health Professionals Second Edition Penny Webb Senior Research Fellow and Head, Gynaecological Cancers Group, Queensland Institute of Medical Research, Brisbane, Australia and Chris Bain Reader in Epidemiology, School of Population Health, University of Queensland, Brisbane, Australia
University Printing House, Cambridge CB2 8BS, United Kingdom Cambridge University Press is part of the University of Cambridge. It furthers the University s mission by disseminating knowledge in the pursuit of education, learning and research at the highest international levels of excellence. Information on this title: /9780521177313 C P. Webb and C. Bain 2011 This publication is in copyright. Subject to statutory exception and to the provisions of relevant collective licensing agreements, no reproduction of any part may take place without the written permission of Cambridge University Press. First published 2011 5th printing 2015 Printed in the United Kingdom by Bell and Bain Ltd, Glasgow A catalogue record for this publication is available from the British Library Library of Congress Cataloguing in Publication data Webb, Penny, 1963 Essential epidemiology : an introduction for students and health professionals /. 2nd ed. p. ; cm. Includes bibliographical references and index. ISBN 978-0-521-17731-3 (pbk.) 1. Epidemiology. I. Bain, Chris, 1947 II. Title. [DNLM: 1. Epidemiologic Methods. 2. Epidemiology. WA 105] RA651.W385 2011 614.4 dc22 2010040293 ISBN 978-0-521-17731-3 Paperback Additional resources for this publication at /webb Cambridge University Press has no responsibility for the persistence or accuracy of URLs for external or third-party internet websites referred to in this publication, and does not guarantee that any content on such websites is, or will remain, accurate or appropriate. Every effort has been made in preparing this book to provide accurate and up-to-date information which is in accord with accepted standards and practice at the time of publication. Although case histories are drawn from actual cases, every effort has been made to disguise the identities of the individuals involved. Nevertheless, the authors, editors and publishers can make no warranties that the information contained herein is totally free from error, not least because clinical standards are constantly changing through research and regulation. The authors, editors and publishers therefore disclaim all liability for direct or consequential damages resulting from the use of material contained in this book. Readers are strongly advised to pay careful attention to information provided by the manufacturer of any drugs or equipment that they plan to use.
Contents Foreword Preface 1 Epidemiology is... A case of food poisoning Subdisciplines of epidemiology On epidemics An historical epidemic The beginnings What does epidemiology offer? What do epidemiologists do? A natural experiment Conclusions 2 How long is a piece of string? Measuring disease frequency What are we measuring? The concepts: prevalence and incidence Measuring disease occurrence in practice: epidemiological studies Measuring disease occurrence in practice: using routine data Other measures commonly used in public health Measuring the burden of disease 3 Who, what, where and when? Descriptive epidemiology Case reports and case series Prevalence surveys Routine data collections Sources of summary data Creative use of existing data x xi 1 3 4 6 8 9 17 18 24 24 27 29 30 32 38 45 52 59 68 68 69 71 72 74 76 83 87 v
vi Contents Confidentiality 4 Healthy research: study designs for public health Observational studies Intervention studies A word about ethics 5 Why? Linking exposure and disease Looking for associations Ratio measures (relative risk) Difference measures (attributable risk) Relative risk versus attributable risk: an example Case control studies 6 Heads or tails: the role of chance Random sampling error Confidence intervals (CI) Statistics in epidemiology Statistical versus clinical significance A final word about confidence intervals and p-values 7 All that glitters is not gold: the problem of error Sources of error in epidemiological studies Selection bias Measurement or information error 8 Muddied waters: the challenge of confounding An example of confounding: is alcohol a risk factor for lung cancer? Criteria for a confounder The effects of confounding Control of confounding 92 92 92 94 96 112 117 121 121 122 125 126 128 133 141 143 149 149 153 154 154 156 158 163 165 165 165 166 167 169 169 181 194 194 196 197 198 201 202 208
Contents vii Confounding: the bottom line 9 Reading between the lines: reading and writing epidemiological papers The research question and study design The study sample: selection bias Measuring disease and exposure: measurement bias Confounding Chance Study validity Descriptive studies Writing papers : one swallow doesn t make a summer 10 Who sank the boat? Association and causation What do we mean by a cause? Association versus causation Evaluating causation Evaluating causality in practice: does H. pylori cause stomach cancer? And then what? 11 Assembling the building blocks: reviews and their uses What is a systematic review? Identifying the literature Different types of study Summarising the data Assessment of causality Assessing the quality of a systematic review Making judgements in practice The end result 12 Outbreaks, epidemics and clusters Professor Adrian Sleigh National Centre for Epidemiology and Population Health, Australian National University Outbreaks, epidemics, endemics and clusters Rare disease clusters 218 219 220 221 222 223 226 228 230 230 232 233 234 235 235 237 238 242 243 250 250 251 252 253 253 256 259 265 267 267 272 273 273 276 278 279
viii Contents Epidemiology of infectious diseases A causal model What influences the spread of infectious diseases? Epidemics or outbreaks Investigating outbreaks Epidemic prevention Tuberculosis: a case study Conclusion 13 Watching not waiting: surveillance and epidemiological intelligence The scope of surveillance Types of surveillance Surveillance in practice Evaluation of surveillance 14 Prevention: better than cure? Disease prevention in public health The scope for preventive medicine Strategies for prevention The population attributable fraction as a guide to prevention Prevention in practice Evaluation of preventive interventions in practice A final (cautionary) word Question 15 Early detection: what benefits at what cost? Why screen? The requirements of a screening programme Evaluation of a screening programme The negative consequences of a screening programme 16 A final word... What does the future hold for epidemiology? Where to now? A final word 282 285 286 293 295 299 300 304 304 305 307 309 312 315 321 321 322 323 323 328 331 336 339 341 342 343 343 345 346 349 361 370 371 372 373 375 377 382 385 386
Contents ix Answers to questions Appendix 1: Direct standardisation Appendix 2: Standard populations Appendix 3: Calculating cumulative incidence and lifetime risk from routine data Appendix 4: Indirect standardisation Appendix 5: Calculating life expectancy from a life table Appendix 6: The Mantel Haenszel method for calculating pooled odds ratios Appendix 7: Formulae for calculating confidence intervals for common epidemiological measures 388 404 406 407 409 411 413 416 Glossary Index 419 435
Foreword As a core discipline of public health, epidemiology provides a perspective and methodological approach relevant to all settings requiring rigorous evidence for health and wellbeing. Excellent introductory texts such as this are therefore invaluable to a range of audiences, including students and teachers, practitioners and researchers. The text leads the reader through the history, perspective, concepts and methods of epidemiology and some key public health applications, in a steady, clear fashion. It is nicely paced with worked examples, illustrative questions and tables and frequent practical asides. The style is easy, accessible and not at all dry, which will be particularly valuable for those whose first language is not English. Underlying this approachability, however, is a strong methodological rigour, reflecting the wide international research experience of the authors that informs their writing and teaching. The second edition of this highly successful book is fully updated and includes expanded sections dealing with new terminology, current themes such as genetic epidemiology, life expectancy and global burden, and the importance of systematic reviews as a key public health tool for assessing causality and setting policy. It will be an excellent reader or background text for undergraduate and graduate students in epidemiology and public health. Carol Brayne Professor of Public Health Medicine University of Cambridge x
Preface Preface to the first edition This book has grown out of our collective experience of teaching introductory epidemiology both in the classroom and to distance students enrolled in public health and health studies programmes in the School of Population Health (formerly the Department of Social and Preventive Medicine), University of Queensland. It began life as a detailed set of course notes that we wrote because we could not find a single epidemiology text that covered all of the areas we felt were important in sufficient detail. As the notes were to be used primarily by distance students, we tried hard to make them accessible with lots of examples, minimal jargon and equations, and by engaging readers in doing epidemiology along the way. Feedback from students and colleagues convinced us that the notes were both approachable and practical. We have built on this, and offer this text as a practical introduction to epidemiology for those who need an understanding of health data they meet in their everyday working lives, as well as for those who wish to pursue a career in epidemiology. Acknowledgements If we were to name everyone who had contributed in some way to this book the list would be endless. We would, however, like to acknowledge some of the great teachers (and their books) from whom we have learned most of what we know, and the books we have relied heavily on for our teaching. These include Brian MacMahon (Epidemiology: Principles and Methods, MacMahon and Pugh, 1970), Olli Miettinen, Charlie Hennekens (Epidemiology in Medicine, Hennekens and Buring, 1987), Ken Rothman (Modern Epidemiology, 1986), Foundations of Epidemiology (Lilienfeld and Lilienfeld, 1980), and Epidemiology (Gordis, 1996). We would also like to thank our colleagues and friends, especially the Fellows from the NHMRC Capacity Grant in Longitudinal Study Methods in the School of Population Health, University of Queensland, and the staff and students from the Cancer and Population Studies Group at the Queensland Institute of Medical xi
xii Preface Research who willingly read drafts of the text and whose constructive feedback helped shape the final version. Particular thanks go to Adrian Sleigh (Australian National University) who authored Chapters 4 [Chapter 13 in the second edition] and 12 and also contributed to Chapter 15 [Chapter 16 in the second edition], Susan Jordan (QIMR) who helped with pulling everything together and Christine Howes (Bristol, UK) who drew all otherwise non-attributed illustrations. Finally, we would like to acknowledge the School of Population Health, University of Queensland, which provided the intellectual environment that led to this book as well as financial support to cover the costs of preparing the final draft. Preface to the second edition This first revision of our text reflects evolution, not revolution. We have listened to the feedback we have received from instructors and students and have tried to simplify and clarify some of the trickier bits of the original text while maintaining a very hands-on approach. We have added new material to reflect contemporary epidemiological practice in public health and have re-ordered some of the existing elements to improve the flow and enhance the continuity between chapters. New and expanded topics include a look at how we measure the burden of disease, greater discussion of issues relevant to ethics and privacy, and appendices covering life tables and calculation of confidence intervals for common epidemiological measures. We have also added a glossary and developed an accompanying website with useful materials including additional test questions and answers, resources for teachers and useful links to a variety of webbased data sources and other epidemiological sites. The website can be accessed at /webb. Our overall aims are, however, unchanged to show the role of epidemiology across a broad range of health monitoring and research activities and to give students a good understanding of the fundamental principles common to all areas of epidemiology including the study of both infectious and chronic diseases as well as public health and clinical epidemiology. To this end, we have maintained the general structure of the original text. As previously, Chapter 1 is a general introduction that both answers the question what is epidemiology and what can it do? and presents the main concepts that are the focus of the rest of the book. Description Chapters 2 3 Association Chapters 4 5 Alternative explanations Chapters 6 8 Integration & interpretation Chapters 9 11 Practical applications Chapters 12 15
Preface xiii The first sections cover the basic principles and underlying theory of epidemiology in a very hands-on way. We start by looking at how we can measure disease and, new to this edition, the overall burden of disease in a population (Chapter 2), followed by a look at the role of descriptive epidemiology in describing health patterns (Chapter 3). We move on to look at the types of study that we use to identify potential causes of disease including an expanded discussion of the potential of record linkage (Chapter 4) and how we quantify the associations between cause and outcome (Chapter 5). In response to feedback from the first edition, we then present a separate look at the role of chance in epidemiology (Chapter 6), a simplified discussion of the thorny issue of error and bias (Chapter 7) and a practical overview of the problem of confounding (Chapter 8). This leads to the next section where we integrate this information in a practical look at how we read and interpret epidemiological reports (Chapter 9), think about assessing causality (Chapter 10) and finally synthesise a mass of information in a single review (Chapter 11). In the final section we look at some specific applications of epidemiology including the study of outbreaks (Chapter 12), surveillance (Chapter 13), prevention including an expanded discussion of how we can assess the impact of different preventive interventions on the health of a population (Chapter 14), and screening (Chapter 15), while Chapter 16 concludes with a fresh look at what epidemiology is and what it can do to help address the health concerns facing the world today. Symbols Throughout the book we have used bold typeface to indicate terms included in the glossary and the following symbols are used to define key elements within the text. We strongly believe that the best way to learn anything is by actually doing it and so have included questions within the text for those who like to test their understanding as they go. Because we also know how frustrating it is to have to search for answers, we have provided these immediately following the questions for those in a hurry to proceed. We have used numerous real-life examples from all around the world to illustrate the key points and to provide additional insights in some areas. Extra examples that provide added interest and complement the main message in the text are given in boxes featuring this symbol. Many books present clinical epidemiology as a separate discipline from public health epidemiology a distinction that is strengthened by the fact that clinical epidemiologists have developed their own names for many standard
xiv Preface epidemiological terms. In practice all epidemiology is based on the same underlying principles, so we have integrated the two approaches throughout the book but have also highlighted specific examples more relevant to the clinical situation. (Please note that this book does not offer a comprehensive coverage of clinical epidemiology; rather we aim to show the similarity of the two areas where they overlap.) We have deliberately tried to keep the main text free of unnecessary detail and equations but have included some epidemiological extras. This material is not essential to the continuity of the core text but provides some additional information for those who like to see where things have come from or want a more detailed perspective. Acknowledgements for the second edition We are again indebted to the many people who have provided input at all stages of the development of this book. In addition to those named previously, a few deserve a special mention. Our former colleague and co-author of the first edition, Sandi Pirozzo, has moved on to a rewarding new career post-epidemiology; we remain grateful for her prior contributions and for her continuing friendship and interest. Adrian Sleigh has kindly updated the chapter on Outbreaks that he wrote for the first edition and has also contributed valuable insights to the chapter on Surveillance and the final chapter. Discernible improvements in the cohesion and internal sign-posting of the book reflect excellent critiques and suggestions we received from Michael O Brien and Kate Van Dooren, the former an educator and the latter a doctoral student within the School of Population Health. Kate also provided much practical support which enabled this revision. Finally, our expanded consideration of the Burden of Disease approach has benefited from interactions with, and teaching materials developed by members of the Burden of Disease group at the School of Population Health, especially Theo Voss, Steven Begg and Alan Lopez. Finally we thank the many users of the first edition, particularly the team from Otago University in New Zealand, who provided the critical feedback that has directly led to this new and hopefully improved edition.