Interpreting Evidence

Bernard Robertson, Barrister, Inner Temple and New ZealandBernard Robertson is a graduate of Oxford, of the LSE, and of the National Police Staff College, Bramshill as well as being a Barrister of the Inner Temple and of New Zealand. After ten years in the Metropolitan Police, Bernard emigrated to New Zealand and taught the law of evidence at undergraduate and postgraduate levels. He and Professor Vignaux have written numerous papers on Bayesian analysis of scientific evidence and of evidence and court decision making in general. He has also been editor of The New Zealand Law Journal and of The New Zealand Law Reports, ensuring that he has kept in touch with a wide range of legal issues, including civil litigation. G. A.Vignaux, Emeritus Professor,?Victoria University, New ZealandG A (Tony) Vignaux (retired) was a physicist and an Operations Research worker, and latterly a Professor of Operations Research at Victoria University. Prior to starting work on legal questions he published on the use of Bayesian methods in physics. With Bernard Robertson he has been invited to address conferences and has been consulted on several legal cases and by enquiries into miscarriage of justice. He has participated in teaching evidence and forensic science to law students at postgraduate level. Charles Berger, Principal Scientist,?Netherlands Forensic Institute,?The NetherlandsCharles Berger is principal scientist at the Netherlands Forensic Institute (NFI), and professor of Criminalistics at Leiden University. He specializes in subjects such as evidence interpretation and forensic inference. At the NFI he is active in a number of areas such as education and research about which he publishes internationally. He also supports the NFI experts, advises the management and oversees scientific quality. He is involved in promoting logically correct reasoning and concluding, introducing more objective methods, and interpretation at the activity level. For such improvements it is essential to explain them as often and as well as possible to all the stakeholders in the justice system. It is an exciting challenge at the interfaces of the worlds of science, police, and law.

• Preface to the First Edition xiPreface to the Second Edition xv1. Introduction 11.1 Three ‘principles’ 11.2 Dreyfus, Bertillon, and Poincaré 31.3 Requirements for Forensic Scientific Evidence 51.3.1 Reliability 61.4 What We Will Cover 62. Interpreting Scientific Evidence 92.1 Relevance and Probative Value 92.1.1 Ideal and Useless Evidence 102.1.2 Typical Evidence 112.1.3 An Aside on Probability and Odds 112.1.4 A Breath-Testing Device 132.2 The Likelihood Ratio and Bayes’ Theorem 142.2.1 The Likelihood Ratio 142.2.2 Bayes’ Theorem 152.2.3 The Effect of Prior Odds 162.2.4 An HIV Test 162.2.5 Transposing the Conditional 172.2.6 Giving Evidence 182.3 Admissibility and Relevance 192.3.1 Prejudging the Case? 202.4 Case Studies 212.4.1 A Useful Presentation of DNA Evidence 212.4.2 The Shoe Mark at the Murder Scene 222.4.3 The Probability of Paternity 232.4.4 Child Sexual Abuse 262.5 Summary 273. The Alternative Hypothesis 293.1 Some Symbols 293.1.1 Hypotheses 293.1.2 Evidence 303.1.3 Probability 303.2 Which Alternative Hypothesis? 303.2.1 Probative Value and the Alternative Hypothesis 303.2.2 Selecting the Appropriate Alternative Hypotheses 313.2.3 Example 323.3 Exclusive, Exhaustive, and Multiple Hypotheses 333.3.1 Exclusiveness 333.3.2 Exhaustiveness 343.3.3 Multiple Hypotheses 353.4 Immigration and Paternity Cases 353.4.1 No Alternative Father 363.4.2 A Named Alternative Father 363.4.3 An Older Example 373.5 ‘It Was My Brother’ 383.6 Traces at the Scene and Traces on the Suspect 393.6.1 Traces at the Scene 393.6.2 Traces on the Accused 393.6.3 The Accused’s Race 403.7 Hypothetical Questions 403.8 Pre-Trial Conferences and Defence Notice 423.9 Case Studies 433.9.1 Alternative Hypotheses in Cases of Child Sexual Abuse 433.9.2 The Shoe Mark Case Again 433.9.3 Sally Clark 443.10 Summary 454. What Questions Can the Expert Deal With? 474.1 The Hierarchy of Propositions 474.2 The Ultimate Issue Rule 504.2.1 Rationale 514.2.2 Experts Must Not Give Evidence on Legal Concepts 514.2.3 The Rule and Logical Inference 524.2.4 The Ultimate Issue Rule Is Correct 534.3 Summary 545. Explaining the Strength of Evidence 555.1 Explaining the Likelihood Ratio 565.1.1 Sensitivity Tables 575.2 The Weight of Evidence 575.3 Words Instead of Numbers? 585.3.1 Standardising Word Meanings 595.3.2 The Inconsistent Meanings of ‘Consistent’ 605.3.3 ‘Could Have’ and ‘Could Have Not’ 615.3.4 There’s Nothing Special about Being ‘Unique’ 615.3.5 ‘Reliability’ 625.3.6 Other Words to Avoid 635.4 Dealing with Wrongly Expressed Evidence 635.5 Case Studies 645.5.1 Shoe Marks 645.5.2 Stomach Contents 665.5.3 Hair Growth 665.6 Summary 676. The Case as a Whole 696.1 Combining Evidence 696.1.1 Dependent and Independent Evidence 706.1.2 Conditional Independence 716.1.3 Combining Dependent Evidence 726.2 Can Combined Weak Evidence Be Stronger Than Its Components? 726.3 The Standard of Proof and the Cost of Errors 746.3.1 Civil Cases 756.3.2 Criminal Cases 756.3.3 Child Sex-Abuse Cases 756.3.4 Is a Quantifiable Doubt a Reasonable Doubt? 756.3.5 What If the Scientific Evidence Is the Only Evidence? 766.4 Assessing Prior Odds 766.4.1 Prior Odds and the Presumption of Innocence 776.5 The Defence Hypothesis and the Prior Odds 786.6 Case Studies 786.6.1 A Bomb-Hoax Call 786.6.2 Loveridge v Adlam 816.7 Summary 827. Forensic Science Methodology 857.1 A General Methodology for Comparative Analysis 867.1.1 Choosing Features 867.1.2 Choosing How to Compare Features 877.1.3 Calculating Same-Source and Different-Source Comparison Scores 887.1.4 Generating Likelihood Ratios 907.2 Assessing the Performance of an Expert or a Comparison System 907.2.1 Discrimination 917.2.2 Calibration 917.2.3 Misleading Evidence 927.2.4 Discrimination versus Calibration 937.2.5 Improving Calibration 937.3 System Performance Characteristics 957.3.1 Tippett Plots 957.3.2 Measuring Discrimination and Calibration Separately 967.4 Case Assessment and Interpretation (CAI) 987.4.1 Defining the Customer Requirement 987.4.2 Assessing How Forensic Science Can Help 997.4.3 Agreeing on a Case Examination Strategy 997.4.4 Examination, Interpretation, and Communication 997.4.5 Case Example, Murder or Suicide? 1007.5 Context Bias 1027.5.1 Base Rate Information 1027.5.2 Case Information 1037.5.3 Reference Material 1037.5.4 Questioned Material 1037.6 Summary 1048. Assigning Likelihood Ratios 1078.2 Glass Refractive Index 1118.3 Colour Comparison 1138.3.1 Colour Feature Selection or Construction 1138.3.2 Colour Comparison Algorithm 1148.3.3 Colour Feature and Score Distribution for Collection 1148.4 Fingerprints 1168.4.1 Feature Selection or Construction 1178.4.2 Comparison Algorithm, and Within- and Between-Source Scores 1198.5 Signatures 1218.6 Psychological Evidence 1258.6.1 The Probative Value of Psychological Evidence 1258.7 Summary 1279. Errors of Thinking 1299.1 A Brace of Lawyers’ Fallacies 1299.1.1 The Prosecutor’s Fallacy 1299.1.2 The Defence Attorney’s Fallacy 1339.1.3 Balance 1349.2 Double-Counting Evidence? 1349.3 The Accuracy and Reliability of Scientific Evidence 1359.3.1 Honest Reporting 1369.3.2 Quality Control 1369.3.3 Laboratory Error Rate 1379.4 Case Studies 1389.4.1 The mad Earl of Ferrers 1389.4.2 The Blood on the Belt 1399.4.3 Broken Glass 1419.5 Summary 14410. Frequentist Statistics and Database Matching 14710.1 The Frequentist Statistical Approach 14810.1.1 Problems of Significance Testing 14810.1.2 What Is a Confidence Interval? 15010.2 Databases 15210.2.1 Using This Evidence 15310.2.2 Traps with Databases 15310.3 The Right Questions and the Wrong Questions 15410.3.1 When the Wrong Questions Give the Right Answers 15510.4 Summary 15811. Implications for the Legal System 16111.1 What Is Expert Evidence? 16111.1.1 Is Expert Evidence Just Opinion Evidence? 16211.1.2 Is ‘Expert Opinion’ Different from ‘Lay Opinion’? 16311.1.3 Expert Evidence as a Subject in Itself 16311.2 Who Is an Expert? 16411.2.1 An Organised Body of Knowledge? 16511.2.2 Forensic Scientists as Expert Witnesses 16611.3 Insanity and the Ultimate Issue Rule 16611.3.1 Is Forensic Science Different from Other Sciences? 16811.4 Novel Forms of Scientific Evidence 16811.4.1 Additional Requirements for Forensic Scientific Evidence? 16811.4.2 The End of the Frye Test – Daubert 17011.4.3 Testing of the Theory or Technique 17111.4.4 Publication and Peer Review 17211.4.5 Actual or Potential Error Rates 17211.4.6 Wide Acceptance 17311.4.7 Conclusions on Daubert 17411.5 Knowledge of Context 17411.5.1 The Importance of Context 17411.5.2 Defence Disclosure 17511.6 Court-Appointed Experts 17611.7 Summary 17712. Conclusion 17912.1 Forensic Science as a Science 18012.2 Conclusions 18112.3 The Fundamental Questions 181Appendix 183A.1 Probability, Odds, Bayes’ Rule and the Weight of Evidence 183A.1.1 Probability 183A.1.2 Odds 184A.1.3 Symbols 185A.2 Laws of Probability 186A.3 The Weight of Evidence 190Index 193