Table Of Content(cid:2)
Interpreting Evidence
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Interpreting Evidence
Evaluating Forensic Science in the Courtroom
Second Edition
by
BERNARD ROBERTSON
Barrister, Wellington, New Zealand
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G.A. VIGNAUX
Emeritus Professor
Victoria University, Wellington, New Zealand
CHARLES E.H. BERGER
Principal Scientist, Netherlands Forensic Institute
Professor of Criminalistics, Leiden University,
The Netherlands
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Thiseditionfirstpublished2016©2016byJohnWiley&Sons,Ltd
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Contents
PrefacetotheFirstEdition xi
PrefacetotheSecondEdition xv
1. Introduction 1
1.1 Three‘principles’ 1
1.2 Dreyfus,Bertillon,andPoincaré 3
1.3 RequirementsforForensicScientificEvidence 5
1.3.1 Reliability 6
1.4 WhatWeWillCover 6
2. InterpretingScientificEvidence 9
2.1 RelevanceandProbativeValue 9
2.1.1 IdealandUselessEvidence 10
2.1.2 TypicalEvidence 11
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2.1.3 AnAsideonProbabilityandOdds 11
2.1.4 ABreath-TestingDevice 13
2.2 TheLikelihoodRatioandBayes’Theorem 14
2.2.1 TheLikelihoodRatio 14
2.2.2 Bayes’Theorem 15
2.2.3 TheEffectofPriorOdds 16
2.2.4 AnHIVTest 16
2.2.5 TransposingtheConditional 17
2.2.6 GivingEvidence 18
2.3 AdmissibilityandRelevance 19
2.3.1 PrejudgingtheCase? 20
2.4 CaseStudies 21
2.4.1 AUsefulPresentationofDNAEvidence 21
2.4.2 TheShoeMarkattheMurderScene 22
2.4.3 TheProbabilityofPaternity 23
2.4.4 ChildSexualAbuse 26
2.5 Summary 27
3. TheAlternativeHypothesis 29
3.1 SomeSymbols 29
3.1.1 Hypotheses 29
3.1.2 Evidence 30
3.1.3 Probability 30
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3.2 WhichAlternativeHypothesis? 30
3.2.1 ProbativeValueandtheAlternativeHypothesis 30
3.2.2 SelectingtheAppropriateAlternativeHypotheses 31
3.2.3 Example 32
3.3 Exclusive,Exhaustive,andMultipleHypotheses 33
3.3.1 Exclusiveness 33
3.3.2 Exhaustiveness 34
3.3.3 MultipleHypotheses 35
3.4 ImmigrationandPaternityCases 35
3.4.1 NoAlternativeFather 36
3.4.2 ANamedAlternativeFather 36
3.4.3 AnOlderExample 37
3.5 ‘ItWasMyBrother’ 38
3.6 TracesattheSceneandTracesontheSuspect 39
3.6.1 TracesattheScene 39
3.6.2 TracesontheAccused 39
3.6.3 TheAccused’sRace 40
3.7 HypotheticalQuestions 40
3.8 Pre-TrialConferencesandDefenceNotice 42
3.9 CaseStudies 43
3.9.1 AlternativeHypothesesinCasesofChildSexualAbuse 43
3.9.2 TheShoeMarkCaseAgain 43
3.9.3 SallyClark 44
(cid:2) 3.10 Summary 45 (cid:2)
4. WhatQuestionsCantheExpertDealWith? 47
4.1 TheHierarchyofPropositions 47
4.2 TheUltimateIssueRule 50
4.2.1 Rationale 51
4.2.2 ExpertsMustNotGiveEvidenceonLegalConcepts 51
4.2.3 TheRuleandLogicalInference 52
4.2.4 TheUltimateIssueRuleIsCorrect 53
4.3 Summary 54
5. ExplainingtheStrengthofEvidence 55
5.1 ExplainingtheLikelihoodRatio 56
5.1.1 SensitivityTables 57
5.2 TheWeightofEvidence 57
5.3 WordsInsteadofNumbers? 58
5.3.1 StandardisingWordMeanings 59
5.3.2 TheInconsistentMeaningsof‘Consistent’ 60
5.3.3 ‘CouldHave’and‘CouldHaveNot’ 61
5.3.4 There’sNothingSpecialaboutBeing‘Unique’ 61
5.3.5 ‘Reliability’ 62
5.3.6 OtherWordstoAvoid 63
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5.4 DealingwithWronglyExpressedEvidence 63
5.5 CaseStudies 64
5.5.1 ShoeMarks 64
5.5.2 StomachContents 66
5.5.3 HairGrowth 66
5.6 Summary 67
6. TheCaseasaWhole 69
6.1 CombiningEvidence 69
6.1.1 DependentandIndependentEvidence 70
6.1.2 ConditionalIndependence 71
6.1.3 CombiningDependentEvidence 72
6.2 CanCombinedWeakEvidenceBeStrongerThanItsComponents? 72
6.3 TheStandardofProofandtheCostofErrors 74
6.3.1 CivilCases 75
6.3.2 CriminalCases 75
6.3.3 ChildSex-AbuseCases 75
6.3.4 IsaQuantifiableDoubtaReasonableDoubt? 75
6.3.5 WhatIftheScientificEvidenceIstheOnlyEvidence? 76
6.4 AssessingPriorOdds 76
6.4.1 PriorOddsandthePresumptionofInnocence 77
6.5 TheDefenceHypothesisandthePriorOdds 78
6.6 CaseStudies 78
(cid:2) 6.6.1 ABomb-HoaxCall 78 (cid:2)
6.6.2 LoveridgevAdlam 81
6.7 Summary 82
7. ForensicScienceMethodology 85
7.1 AGeneralMethodologyforComparativeAnalysis 86
7.1.1 ChoosingFeatures 86
7.1.2 ChoosingHowtoCompareFeatures 87
7.1.3 CalculatingSame-SourceandDifferent-SourceComparison
Scores 88
7.1.4 GeneratingLikelihoodRatios 90
7.2 AssessingthePerformanceofanExpertoraComparisonSystem 90
7.2.1 Discrimination 91
7.2.2 Calibration 91
7.2.3 MisleadingEvidence 92
7.2.4 DiscriminationversusCalibration 93
7.2.5 ImprovingCalibration 93
7.3 SystemPerformanceCharacteristics 95
7.3.1 TippettPlots 95
7.3.2 MeasuringDiscriminationandCalibrationSeparately 96
7.4 CaseAssessmentandInterpretation(CAI) 98
7.4.1 DefiningtheCustomerRequirement 98
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7.4.2 AssessingHowForensicScienceCanHelp 99
7.4.3 AgreeingonaCaseExaminationStrategy 99
7.4.4 Examination,Interpretation,andCommunication 99
7.4.5 CaseExample,MurderorSuicide? 100
7.5 ContextBias 102
7.5.1 BaseRateInformation 102
7.5.2 CaseInformation 103
7.5.3 ReferenceMaterial 103
7.5.4 QuestionedMaterial 103
7.6 Summary 104
8. AssigningLikelihoodRatios 107
8.1 DNA 108
8.1.1 ASingleComparisonwithaMatchasaResult 109
8.1.2 ADatabaseSearchwithaSingleMatchasaResult 109
8.1.3 ADatabaseSearchwithMultipleMatchesasaResult 110
8.1.4 ExtremelyLargeLRs 111
8.2 GlassRefractiveIndex 111
8.3 ColourComparison 113
8.3.1 ColourFeatureSelectionorConstruction 113
8.3.2 ColourComparisonAlgorithm 114
8.3.3 ColourFeatureandScoreDistributionforCollection 114
8.4 Fingerprints 116
(cid:2) 8.4.1 FeatureSelectionorConstruction 117 (cid:2)
8.4.2 ComparisonAlgorithm,andWithin-andBetween-Source
Scores 119
8.5 Signatures 121
8.6 PsychologicalEvidence 125
8.6.1 TheProbativeValueofPsychologicalEvidence 125
8.7 Summary 127
9. ErrorsofThinking 129
9.1 ABraceofLawyers’Fallacies 129
9.1.1 TheProsecutor’sFallacy 129
9.1.2 TheDefenceAttorney’sFallacy 133
9.1.3 Balance 134
9.2 Double-CountingEvidence? 134
9.3 TheAccuracyandReliabilityofScientificEvidence 135
9.3.1 HonestReporting 136
9.3.2 QualityControl 136
9.3.3 LaboratoryErrorRate 137
9.4 CaseStudies 138
9.4.1 ThemadEarlofFerrers 138
9.4.2 TheBloodontheBelt 139
9.4.3 BrokenGlass 141
9.5 Summary 144
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Contents ix
10. FrequentistStatisticsandDatabaseMatching 147
10.1 TheFrequentistStatisticalApproach 148
10.1.1 ProblemsofSignificanceTesting 148
10.1.2 WhatIsaConfidenceInterval? 150
10.2 Databases 152
10.2.1 UsingThisEvidence 153
10.2.2 TrapswithDatabases 153
10.3 TheRightQuestionsandtheWrongQuestions 154
10.3.1 WhentheWrongQuestionsGivetheRightAnswers 155
10.4 Summary 158
11. ImplicationsfortheLegalSystem 161
11.1 WhatIsExpertEvidence? 161
11.1.1 IsExpertEvidenceJustOpinionEvidence? 162
11.1.2 Is‘ExpertOpinion’Differentfrom‘LayOpinion’? 163
11.1.3 ExpertEvidenceasaSubjectinItself 163
11.2 WhoIsanExpert? 164
11.2.1 AnOrganisedBodyofKnowledge? 165
11.2.2 ForensicScientistsasExpertWitnesses 166
11.3 InsanityandtheUltimateIssueRule 166
11.3.1 IsForensicScienceDifferentfromOtherSciences? 168
11.4 NovelFormsofScientificEvidence 168
11.4.1 AdditionalRequirementsforForensicScientificEvidence? 168
(cid:2) 11.4.2 TheEndoftheFryeTest – Daubert 170 (cid:2)
11.4.3 TestingoftheTheoryorTechnique 171
11.4.4 PublicationandPeerReview 172
11.4.5 ActualorPotentialErrorRates 172
11.4.6 WideAcceptance 173
11.4.7 ConclusionsonDaubert 174
11.5 KnowledgeofContext 174
11.5.1 TheImportanceofContext 174
11.5.2 DefenceDisclosure 175
11.6 Court-AppointedExperts 176
11.7 Summary 177
12. Conclusion 179
12.1 ForensicScienceasaScience 180
12.2 Conclusions 181
12.3 TheFundamentalQuestions 181
Appendix 183
A.1 Probability,Odds,Bayes’RuleandtheWeightofEvidence 183
A.1.1 Probability 183
A.1.2 Odds 184
A.1.3 Symbols 185
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A.2 LawsofProbability 186
A.2.1 Complementarity 186
A.2.2 ProductRule 186
A.2.3 SumRule 187
A.2.4 TheLikelihoodRatio,LR 188
A.2.5 Bayes’Rule 188
A.2.6 ProbabilityForm 188
A.2.7 OddsFormofBayes’Rule 189
A.2.8 CombiningEvidence 189
A.3 TheWeightofEvidence 190
Index 193
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Preface to the First Edition
Thisbookstartedaspartofawiderproject,theexaminationoftheapplicabilityoflogical
and probabilistic reasoning to evidence generally. This has been the subject of vigorous
discussion in the legal literature and is one of the main threads of the ‘New Evidence
Scholarship’.
Forensicsciencesuggesteditselfasacasestudyasthereseemedtobesomedegreeof
consensusthatforensicscientificevidenceshouldbethoughtaboutinprobabilisticterms,
butwhenwesurveyedthefielditappearedtobeamess.
• Someexpertwitnesses,suchasfingerprintofficers,makecategoricalstatementsthattwo
impressionsarefromthesamefinger.
• Some experts, such as glass experts, would only say that a sample could have come
from a particular source and then gave some straightforward sounding statistics about
thefrequencyofglassofthattype.
• Sometypesofevidence,suchasDNA,seemedtoinvolvestatisticalargumentsofimpen-
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etrablecomplexity.
Thelawseemedinequalconfusion.
• Therewasarulepreventinggivinganopinionontheultimateissue,yetcourtsregularly
heardwitnessestalkabouttheprobabilityofpaternity.
• Acourtwouldrejectevidenceinonecasebecauseitusurpedtheroleofthejuryandin
anotherbecauseitwasnotdefinitiveandconclusive.
• Courtssometimespointedoutproblemswithevidencethattheforensicscienceprofes-
sion did littleabout and sometimes ruled evidence out for reasons that had littleto do
withitsprobativevalue.
Italsoseemedtousthatcourtsandtextbookwriterswerekeenertobandywordssuch
as‘reliability’and‘regardtheevidencewithcaution’thantoexplainwhatideaslaybehind
thesephrases.
Thetimehadclearlycomeforsomefundamentalre-evaluationofforensicscience.As
we studied the matter, we realised that the application of a few basic logical principles
solvedtheproblemsofdescriptionandprescriptionwithwhichwewerefaced.Thatisnot
tosaythatsolutionscameeasily;theapplicationoftheseprinciplesrequireshardthinking
andwecannotpretendtoofferanswerstoallthequestions.Theresultsleadtoadviceabout
howtothinkaboutevidenceofmuchmorepracticalvaluethananadmonitionto‘regard
theevidencewithcaution’.
While preparing this book we found some forensic scientists who had been thinking
alongthesamelinesandhadpublishedpapersinthescientificliterature.Themostprolific
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