MATH ON TRIAL How Numbers Get Used and Abused in the Courtroom Leila Schneps and Coralie Colmez BASIC BOOKS A Member of the Perseus Books Group • New York Copyright © 2013 by Leila Schneps and Coralie Colmez Published by Basic Books, A Member of the Perseus Books Group All rights reserved. No part of this book may be reproduced in any manner whatsoever without written permission except in the case of brief quotations embodied in critical articles and reviews. For information, address Basic Books, 250 West 57th Street, 15th Floor, New York, NY 10107-1307. Books published by Basic Books are available at special discounts for bulk purchases in the United States by corporations, institutions, and other organizations. For more information, please contact the Special Markets Department at the Perseus Books Group, 2300 Chestnut Street, Suite 200, Philadelphia, PA 19103, or call (800) 810-4145, ext. 5000, or e-mail
[email protected] Designed by Jeff Williams Library of Congress Cataloging-in-Publication Data Schneps, Leila. Math on trial : how numbers get used and abused in the courtroom / Leila Schneps and Coralie Colmez. pages cm Includes bibliographical references and index. ISBN 978-0-465-03794-0 (e-book) 1. Forensic statistics. I. Colmez, Coralie, 1988– II. Title. K2290.S73S36 2013 345'.07—dc23 2012040624 10 9 8 7 6 5 4 3 2 1 We dedicate this book to all those who have suffered from miscarriages of justice, and to all victims of crimes whose perpetrators went unpunished, due to the misuse or misunderstanding of mathematics in the legal process. CONTENTS Introduction MATH ERROR NUMBER 1 » MULTIPLYING NON-INDEPENDENT PROBABILITIES The Case of Sally Clark: Motherhood Under Attack MATH ERROR NUMBER 2 » UNJUSTIFIED ESTIMATES The Case of Janet Collins: Hairstyle Probability MATH ERROR NUMBER 3 » TRYING TO GET SOMETHING FROM NOTHING The Case of Joe Sneed: Absent from the Phone Book MATH ERROR NUMBER 4 » DOUBLE EXPERIMENT The Case of Meredith Kercher: The Test That Wasn’t Done MATH ERROR NUMBER 5 » THE BIRTHDAY PROBLEM The Case of Diana Sylvester: Cold Hit Analysis MATH ERROR NUMBER 6 » SIMPSON’S PARADOX The Berkeley Sex Bias Case: Discrimination Detection MATH ERROR NUMBER 7 » THE INCREDIBLE COINCIDENCE The Case of Lucia de Berk: Carer or Killer? MATH ERROR NUMBER 8 » UNDERESTIMATION The Case of Charles Ponzi: American Dream, American Scheme MATH ERROR NUMBER 9 » CHOOSING A WRONG MODEL The Case of Hetty Green: A Battle of Wills MATH ERROR NUMBER 10 » MATHEMATICAL MADNESS The Dreyfus Affair: Spy or Scapegoat? Conclusion Sources Credits Index INTRODUCTION Everywhere we turn we are assailed by numbers. Advertisements, news, price reductions, medical information, weather forecasts, investment, risk assessment: all this and more is communicated to us through probabilities and statistics. But the problem is that these figures are not always used to convey information. As often as not, they are used to give us spin: to influence, frighten, and mislead us with the cool authority of numbers and formulas. Now, you might think this a trivial matter. You may be one of those people who skip past the numbers in the articles you read, who pay no attention to the declarations of sensational increase or decrease in whatever drama is playing out on the front page, whether it be global warming, shark infestations, or illiteracy. At worst, you think, people are mildly misinformed. But as we show in Math on Trial, the misuse of mathematics can be deadly. The same mathematical tricks that mislead the public about market trends and risk and social problems have sent innocent peo