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The Evaluation of Forensic DNA Evidence Committee on DNA Forensic Science: An Update Commission on DNA Forensic Science: An Update National Research Council NATIONAL ACADEMY PRESS Washington, D.C. 1996
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Page ii National Academy Press · 2101 Constitution Ave., N.W. · Washington, D.C. 20418 NOTICE: The project that is the subject of this report was approved by the Governing Board of the National Research Council, whose members are drawn from the councils of the National Academy of Sciences, the National Academy of Engineering, and the Institute of Medicine. The members of the committee responsible for the report were chosen for their special competences and with regard for appropriate balance. In preparing its report, the committee invited people with different perspectives to present their views. Such invitation does not imply endorsement of those views. This report has been reviewed by a group other than the authors according to procedures approved by a Report Review Committee consisting of members of the National Academy of Sciences, the National Academy of Engineering, and the Institute of Medicine. This study by the National Research Council's Virtual Commission on DNA Forensic Science: An Update was sponsored by the National Institute of Justice, the State Justice Institute, the National Science Foundation (Grant No. DMS-9415474), the National Institutes of Health, and the Department of Energy (Grant No. DE-FGO2-94ER61782). Supported under Award # 93-IJ-CX-0008 from the National Institute of Justice, Office of Justice Programs, U.S., Department of Justice. Points of view in this document are those of the authors and do not necessarily represent the official position of the US Department of Justice. Library of Congress Cataloging-in-Publication Data The evaluation of forensic DNA evidence / Committee on DNA Forensic Science: an Update. Commission on DNA Forensic Science: an Update, National Research Council. p. cm. Includes bibliographical references and index. ISBN 0-309-05395-1 1. Forensic genetics. I. National Research Council (U.S.). Committee on DNA Technology in Forensic Science: an Update. II. National Research Council (U.S.). Commission on DNA Technology in Forensic Science: an Update. RA1057.5.E94 1996 614'.1dc20 96-25364 Copyright 1996 by the National Academy of Sciences. All rights reserved. Printed in the United States of America First Printing. December 1996 Second Printing, August 1998
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Page iii Committee on DNA Forensic Science: An Update JAMES F. CROW, Chair, University of Wisconsin, Madison MARGARET A. BERGER, Brooklyn Law School, Brooklyn, New York SHARI S. DIAMOND, University of Illinois at Chicago/American Bar Foundation DAVID H. KAYE, Arizona State University, College of Law, Tempe HAIG H. KAZAZIAN, University of Pennsylvania, Philadelphia ARNO G. MOTULSKY, University of Washington, Seattle THOMAS A. NAGYLAKI, University of Chicago, Chicago, Illinois MASATOSHI NEI, Pennsylvania State University, University Park GEORGE F. SENSABAUGH, University of California, Berkeley DAVID O. SIEGMUND, Stanford University, Stanford, California STEPHEN M. STIGLER, University of Chicago, Chicago, Illinois Advisor VICTOR A. McKUSICK, The Johns Hopkins Hospital, Baltimore, Maryland NRC Staff ERIC A. FISCHER, Study Director LEE R. PAULSON, Senior Staff Officer MIRON L. STRAF, Senior Staff Officer JOHN R. TUCKER, Senior Staff Officer PAULETTE A. ADAMS, Senior Project Assistant NORMAN GROSSBLATT, Editor
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Page iv Commission on DNA Forensic Science: An Update THOMAS D. POLLARD, Chair, The Johns Hopkins University, Baltimore, Maryland PETER J. BICKEL, University of California, Berkeley MICHAEL T. CLEGG, University of California, Riverside BURTON SINGER, Princeton University, Princeton, New Jersey NRC Staff PAUL GILMAN, Executive Director
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Page v Preface DNA analysis promises to be the most important tool for human identification since Francis Galton developed the use of fingerprints for that purpose. We can confidently predict that, in the not-distant future, persons as closely related as brothers will be routinely distinguished, and DNA profiles will be as fully accepted as fingerprints now are. But that time has not yet arrived, and the winds of controversy have not been stilled. Hence this report. The technique for DNA profiling first appeared about 10 years ago, and the subject is still young. In the early days there was doubt, both as to the reproducibility and reliability of the methods and as to the appropriateness of simplistic calculations that took no account of possible subdivision of the population. Despite the potential power of the technique, there were serious reservations about its actual use. In 1989, the National Research Council formed the Committee on DNA Technology in Forensic Science to study this new technique. The committee issued its report in 1992. The report resolved a number of questions, and several of its recommendations were widely adopted. Nevertheless, it generated controversy and criticism. Much of that centered on the "interim ceiling principle." a procedure intended to provide an estimate of a profile frequency that is highly conservative (i.e., favorable to the defendant) and independent of the racial origins of the DNA. The principle was criticized as being arbitrary and unnecessarily conservative, as not taking population genetic theory into account, and as being subject to misuse. In April 1993, Judge William Sessions, then the Director of the FBI, requested
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Page vi that the NRC do a follow-up study to resolve the controversy and to answer other questions that recent empirical work permitted such a study to address. After a meeting of consultants in June 1993, the NRC decided to form a new committee and on August 30, 1993, I was asked to chair it. After a year's delay, mainly due to funding uncertainties, the committee members were named in August 1994, and the first meeting was held in September of that year. Subsequent meetings were held in November and in January and March of 1995. The main recommendations were agreed on and several revisions of a report were prepared between March and June. The remainder of the time has been spent in editing, revising, reviewing by the NRC Report Review Committee, and printing. In the report, after an introduction and background material (Chapters I and 2), we deal with the question of errors in the laboratory and chain of custody and recommend procedures to minimize them (Chapter 3). We then address the question of population subdivision and propose calculating procedures that take it into account (Chapter 4). We also consider the statistical interpretation of DNA evidence, including statistical problems associated with the use of existing databases (Chapter 5). Finally, we include a review of DNA in the courts since the 1992 report (Chapter 6). Specific recommendations are numbered and given at the ends of the chapters and are reproduced in the Executive Summary and Overview. Other statements in the text are not intended to have the force of formal recommendations, although we do make a number of suggestions. We agree with some statements of the 1992 report and disagree with others. Statements that are not discussed are neither endorsed nor rejected. This report is the result of many exchanges by phone, fax, and e-mail among the individual committee members and Eric Fischer. We were greatly assisted throughout the project by Lee Paulson and Paulette Adams. The editor was Norman Grossblatt. During the course of the study we received advice by conversation and correspondence from many people. Some of these are listed at the end of the report, but the list is far from complete. We are indebted to all. We particularly thank those who took part in our public meeting on November 18, 1994. Financial support for this study was provided by the National Institute of Justice, the State Justice Institute, the National Science Foundation, the National Institutes of Health, and the Department of Energy. This is a period of rapid progress in developing and testing new methods of DNA analysis and of rapid increase in the size and diversity of databases. The information that has accumulated since the 1992 report permits us to be more confident of our recommendations. Courts have seen estimates of match probability ranging over several orders of magnitude. Our recommendations should lead to much greater agreement among the various estimates. I have no illusion that our report will eliminate the controversy; remaining uncertainties
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Page vii and the adversary system in the courts guarantee its continuance. But I hope that we have substantially narrowed the range of acceptable differences. JAMES F. CROW CHAIRMAN COMMITTEE ON DNA FORENSIC SCIENCE: AN UPDATE
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Page viii The National Academy of Sciences is a private, nonprofit. self-perpetuating society of distinguished scholars engaged in scientific and engineering research, dedicated to the furtherance of science and technology and to their use for the general welfare. Upon the authority of the charter granted to it by the Congress in 1863, the Academy has a mandate that requires it to advise the federal government on scientific and technical matters. Dr. Bruce Alberts is president of the National Academy of Sciences. The National Academy of Engineering was established in 1964, under the charter of the National Academy of Sciences, as a parallel organization of outstanding engineers. It is autonomous in its administration and in the selection of its members, sharing with the National Academy of Sciences the responsibility for advising the federal government. The National Academy of Engineering also sponsors engineering programs aimed at meeting national needs, encourages education and research, and recognizes the superior achievements of engineers. Dr. William A. Wulf is interim president of the National Academy of Engineering. The Institute of Medicine was established in 1970 by the National Academy of Sciences to secure the services of eminent members of appropriate professions in the examination of policy matters pertaining to the health of the public. The Institute acts under the responsibility given to the National Academy of Sciences by its congressional charter to be an adviser to the federal government and, upon its own initiative, to identify issues of medical care, research, and education. Dr. Kenneth 1. Shine is president of the Institute of Medicine. The National Research Council was organized by the National Academy of Sciences in 1916 to associate the broad community of science and technology with the Academy's purposes of furthering knowledge and advising the federal government. Functioning in accordance with general policies determined by the Academy, the Council has become the principal operating agency of both the National Academy of Sciences and the National Academy of Engineering in providing services to the government, the public, and the scientific and engineering communities. The Council is administered jointly by both Academies and the Institute of Medicine. Dr. Bruce Alberts and Dr. William A. Wulf are chairman and interim vice chairman, respectively, of the National Research Council.
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Page ix Contents Executive Summary 1 Overview 9 Introduction, 9 Our Assignment, 10 DNA Typing, 11 Basic Genetic Principles, 12 Forensic DNA Identification, 14 VNTRs, 14 DNA Profiling, 15 Matching and Binning of VNTRs, 18 Allele (Bin) Frequencies, 21 PCR-Based Systems, 21 Assuring Laboratory Accuracy 23 Population Genetics, 25 Randomly Mating Populations, 26 Population Structure, 28 Dealing with Subpopulations, 28 Persons from the Same Subpopulation 29 Some Statistical Considerations 30 The Reference Database, 30 Match Probability, Likelihood Ratio, and Two Fallacies. 31
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Page x Bayes's Theorem, 31 Suspect Identified by Database Search, 32 Uniqueness, 33 Uncertainty About Estimated Frequencies, 33 PCR-Based Tests, 34 The Ceiling Principles, 35 DNA in the Courts, 36 Conclusions and Recommendations, 36 Admissibility of DNA Evidence (Chapter 2), 36 Laboratory Errors (Chapter 3), 37 Population Genetics (Chapter 4), 38 Statistical Issues (Chapter 5), 39 Legal Issues (Chapter 6), 41 Illustrative Example, 42 A Typical Case, 42 Suspect Found by Searching a Database, 45 Suspect and Evidence from the Same Subpopulation, 46 A PCR-Break System, 46 1 Introduction 47 The 1992 National Research Council Report, 48 The Committee's Task, 49 The Validity of DNA Typing, 50 The Use of DNA for Exclusion, 51 Changes Since the 1992 Report, 52 Population Data, 52 Technical Improvements, 53 Paternity Testing, 53 Regulatory Oversight, 54 Seemingly Contradictory Numbers, 55 Very Small Probabilities, 56 Fingerprints and Uniqueness, 56 Designating Population Groups and Subgroups, 57 The Nature of Our Recommendations, 58 2 Genetic And Molecular Basis Of DNA Typing 60 Fundamentals of Genetics, 60 VNTR Typing, 65 PCR-Based Methods, 69 Conclusions, 73
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Page xi 3 Ensuring High Standards Of Laboratory Performance 75 Quality Control and Quality Assurance in the Laboratory, 76 Current QC and QA Guidelines, 76 The Role of Proficiency-Testing and Audits, 78 Safeguarding Against Error, 80 Sample Mishandling and Data-Recording Errors, 80 Faulty Reagents, Equipment, Controls, or Technique, 82 Evidence Contamination, 82 Analyst Bias, 84 Should an Error Rate Be Included in Calculations?, 85 Retesting, 87 Conclusions and Recommendations, 87 Laboratory Errors, 87 Proficiency Tests, 88 Duplicate Tests, 88 4 Population Genetics 89 Allele and Genotype Proportions, 90 Random Mating and Hardy-Weinberg Proportions, 90 HW Proportions in a Large Sample, 93 Exclusion Power of a Locus, 96 Departures from HW Proportions, 97 Inbreeding and Kinship, 98 Population Subgroups, 99 Subpopulation Theory, 102 Taking Population Structure into Account, 104 Multiple Loci and Linkage Equilibrium, 106 How Much Departure from LE is Expected?, 107 What Do the VNTR Data Show?, 108 Relatives, 113 Persons from the Same Subpopulation, 113 PCR-Based Systems, 116 A Conservative Rule for PCR Loci, 119 Development of New Systems, 120 Inadequate Databases, 121 Conclusions and Recommendations, 122 Evidence DNA and Suspect from the Same Subgroup, 122 Insufficient Data, 123 Dealing with Relatives, 123 Appendix 4A, 123
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Page xii 5 Statistical Issues 125 Data Sources, 126 Match Probability and Likelihood Ratio, 127 Mixed Samples, 129 Bayes's Theorem, 130 Two Fallacies, 133 Suspect Identified by a DNA Database Search, 133 Very Small Probabilities, 135 Uniqueness, 136 Statistical Aspects of VNTR Analysis, 139 Determining a Match, 139 Binning, 142 Confidence Intervals for Match Probabilities, 146 Alleles with Low Frequency, 148 Individual Variability and Empirical Comparisons, 148 Geographical Subdivision, 149 Differences Among Subgroups, 151 Different Races, 151 More Conservative Formulae, 154 The Ceiling Principles, 156 Direct Count from a Database, 159 Conclusions and Recommendations, 160 Statistical Issues, 160 Database Searches, 161 Uniqueness, 161 Matching and Binning, 161 Ceiling Principles, 162 Further Research, 162 Appendix 5a, 162 Appendix 5b, 163 Appendix 5c, 165 6 DNA Evidence In The Legal System 166 Legal Standards and Procedures, 167 The Defendant's Right to Discovery, 167 Expertise, 169 General Acceptance and Sound Methodology, 171 Balancing and Weight, 174 Trends in the Admissibility of DNA Evidence, 174 Typing Methods, 176 VNTR Profiling, 176 PCR-Based Testing, 177
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Page xiii Laboratory Error, 179 Population and Subpopulation Frequencies, 185 Convenience Samples, 186 The Disagreement About Substructure, 187 Ceiling Frequencies in Court, 188 Explaining the Meaning of a Match, 192 The Necessity for Quantitative Estimates, 193 Qualitative Testimony on Uniqueness or Infrequency, 193 Quantitative Assessments: Frequencies and Match Probabilities, 195 Quantitative Assessments: Likelihood Ratios and Posterior Odds, 199 Importance of Behavioral Research, 203 Conclusions, 204 Appendix 6a, 205 Abbreviations 212 Glossary 214 Biographical Information 219 Acknowledgments 224 References 226 Index 241
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Page xv Note to Readers This report includes three parts: Executive Summary. This is intended for those who would like to know our recommendations along with minimal explanations. It will be of interest to those who want to get immediately to the bottom line. After a brief introduction, the conclusions are stated followed by a brief description of the rationale. Overview. This is intended for readers who are interested in the background for the conclusions and recommendations without the technical explanations in the main body of the report. Our intention is to present the scientific background for the conclusions with a minimum of jargon and technical material. The conclusions and recommendations, which are scattered in the various chapters of the main report, are repeated here for the reader's convenience. Chapters 1-6. These provide the scientific and technical background for the conclusions and recommendations. They are intended to be a rather thorough review of the basic principles and of the systems used in forensic analysis. They provide background data supporting the conclusions. Although we have tried to write as clearly as possible, the chapters, especially 4 and 5, require some background in statistics and population genetics. Readers of any of the sections will find it useful to refer to the list of abbreviations (p 212) and the glossary (p 214). We are not aware of any inconsistencies among the three sections, but the full chapters provide the most accurate reflection of the committee's views.
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