Epidemiology Matters Epidemiology Matters A New Introduction to Methodological Foundations KATHERINE M. KEYES Department of Epidemiology, Columbia University SANDRO GALEA Department of Epidemiology, Columbia University 1 1 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. Oxford New York Auckland Cape Town Dar es Salaam Hong Kong Karachi Kuala Lumpur Madrid Melbourne Mexico City Nairobi New Delhi Shanghai Taipei Toronto With offices in Argentina Austria Brazil Chile Czech Republic France Greece Guatemala Hungary Italy Japan Poland Portugal Singapore South Korea Switzerland Thailand Turkey Ukraine Vietnam Oxford is a registered trademark of Oxford University Press in the UK and certain other countries. 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ISBN 978–0–19–933124–6 (alk. paper) I. Galea, Sandro, author. II. Title. [DNLM: 1. Epidemiologic Methods. 2. Epidemiologic Research Design. 3. Epidemiology. WA 950] RA652.2.C55 614.4—dc23 2014001646 This material is not intended to be, and should not be considered, a substitute for medical or other professional advice. Treatment for the conditions described in this material is highly dependent on the individual circumstances. And, while this material is designed to offer accurate information with respect to the subject matter covered and to be current as of the time it was written, research and knowledge about medical and health issues is constantly evolving and dose schedules for medications are being revised continually, with new side effects recognized and accounted for regularly. Readers must therefore always check the product information and clinical procedures with the most up-to-date published product information and data sheets provided by the manufacturers and the most recent codes of conduct and safety regulation. The publisher and the authors make no representations or warranties to readers, express or implied, as to the accuracy or completeness of this material. Without limiting the foregoing, the publisher and the authors make no representations or warranties as to the accuracy or efficacy of the drug dosages mentioned in the material. The authors and the publisher do not accept, and expressly disclaim, any responsibility for any liability, loss or risk that may be claimed or incurred as a consequence of the use and/or application of any of the contents of this material. 9 8 7 6 5 4 3 2 1 Printed in the United States of America on acid-free paper We dedicate this book to our families, who make our work worthwhile. Contents About the Authors ix Acknowledgments xi Prefatory Note xiii 1. An Introduction 1 2. What Is a Population and What Is Population Health? 9 3. What Is an Exposure, What Is a Disease, and How Do We Measure Them? 18 4. What Is a Sample? 33 5. Watching a Sample, Counting Cases 49 6. Are Exposures Associated With Health Indicators? 70 7. What Is a Cause? 91 8. Is the Association Causal, or Are There Alternative Explanations? 113 9. How Do Noncausal Associations Arise? 122 10. How Can We Mitigate Against Noncausal Associations in Design and Analysis? 152 11. When Do Causes Work Together? 173 12. Do the Results Matter Beyond the Study Sample? 189 13. How Do We Identify Disease Early to Minimize Its Consequences? 198 14. Conclusion: Epidemiology and What Matters Most 221 Index 233 About the Authors Katherine M. Keyes, PhD, is Assistant Professor of Epidemiology at Columbia University. Her research focuses on life course epidemiology with particular attention to psychiatric disorders, including cross-generational cohort effects on substance use, mental health, and chronic disease. She has particular expertise in the development and application of novel epidemio- logical methods and in the development of epidemiological theory to mea- sure and elucidate the drivers of population health. Sandro Galea, MD, DrPH, is Chair of the Department of Epidemiology at Columbia University. His work focuses on the social production of health of urban populations, innovative cells-to-society approaches to popula- tion health, and advancing a consequentialist approach to epidemiology in the service of population health. He is a past president of the Society for Epidemiologic Research and an elected member of the Institute of Medicine of the National Academies of Science. Acknowledgments This book could not have been completed without the invaluable assis- tance of numerous students and colleagues. This book stands on the shoul- ders of many epidemiology and population health texts that we consulted throughout the writing of Epidemiology Matters. In particular, the writing of Raj Bhopal, Leon Gordis, Sander Greenland, Anwer Khurshid, Ollie Miettenen, Kenneth Rothman, Hardeo Sahai, Rodolfo Saracci, and Sharon Schwartz have not only been instructive but have provided invaluable guid- ance on how to present and formalize epidemiologic methods. In particular, discussions of epidemiologic methods with Sharon Schwartz have informed the structure and content of this text. Angelina Caruso was the architect of our hypothetical population of Farrlandians and provided editorial assistance on all aspects of the book. Sabrina Hermosilla provided valuable feedback on all of the text and figures and assisted with numerical examples. Linda Kahn assisted with exercises and problem sets for each of the chapters and provided keen editorial insight on the text. Adam Ciarleglio provided biostatistical expertise and editing throughout. All errors and omissions in this book are ours alone. Many of these chapters were piloted in two introductory epide- miology classes: Principles of Epidemiology at Columbia University, and the Epidemiology Core class at the École des hautes études en santé publique in Paris, France. We would like to thank Silvia Martins and Moise Desvarieux for allowing us to use the draft chapters in their courses and for their feed- back on the chapters throughout the semester. We would also like to thank the students in these courses for valuable feedback. This book was primarily written during evenings, weekends, and even during vacations. We would like to thank our families for patiently allowing us the time and space to complete this text and for their encouragement and support along the way. Specifically, KMK would like to thank Jeff and Aidan Wild, and SG would like to thank Margaret Kruk, Oliver Luke Galea, and Isabel Tess Galea. Prefatory Note This book should be read and used in conjunction with material available through our digital space: www.epidemiologymatters.org. “Epidemiology Matters,” the digital space, is intended to engage readers and learners on a continuum of instruction of ever-advancing epidemio- logic methods. Here students and instructors will find material, topics, and examples that expand on the text in each of the chapters and material that complements text in the book. We also provide exercises and practice sets for each of the chapters and slide sets that can be used by lecturers. Answer keys for all exercises are available to instructors by emailing Katherine M. Keyes ([email protected]) or Sandro Galea ([email protected]). Further, we will regularly update the digital space with additional topics that were not covered in this text. Epidemiologymatters.org also provides additional instructional material on a wide variety of epidemiology and population health topics as a resource for researchers and lay professionals interested in developing skills in quantitative population health science. Epidemiology Matters 1 An Introduction Epidemiology is the science of understanding the causes and distribution of population health so that we may intervene to prevent disease and promote health. The definition of epidemiology has not substantially changed since the formal origins of the field more than 150 years ago (Transactions of the Epidemiological Society of London, 1876). Centrally there are, and have always been, two core functions for the field: (a) identifying causes of population health (b) so that we may intervene (Galea, 2013). These two core functions posit a pragmatic vision for the discipline: Epidemiology is the science of population health, aiming to understand the key causes of health and disease and doing so in a way that it may inform intervention so we may act. The combination of scientific inquiry and call to action makes epidemiol- ogy a unique science. Identifying the causes of population health requires an understanding of the nature of populations; the conditions that shape them over time and