To my knowledge, the nc state crime laboratory and the charlottemecklenburg police department do not employ the likelihood ratio in their interpretations of dna evidence, though it could be adopted at some point in the future. Nist experts urge caution in use of courtroom evidence. Statistical evaluation of forensic dna profile evidence. As such, the book introduces terms used to describe dna profiles and profile. Several methods exist for weight of evidence calculations on dna mixtures where dropout is a possibility. We discuss the use of a likelihood ratio lr model, lrmix 1, to evaluate the strength of dna evidence for complex profiles where the crime stain is a mixture.
The committee comments on statements in the original book that proved controversial or that have been misapplied in the courts. Therefore and contrary to this expensive guide for lawyers the likelihood ratio does not show how many times more plausible the prosecution hypothesis is given the dna evidence. The likelihood ratio is particularly well suited for the statistical analysis of forensic dna evidence because, as in a trial, two alternative hypotheses are compared. Many nonspecialists have found this book to be an intuitive introduction to probability concepts. A likelihood approach to dna evidence springerlink. Confidence interval of the likelihood ratio associated with. Testing likelihood ratios produced from complex dna profiles. Pretest and posttest probabilities are also covered. Recent years have seen the advent of other, equally important, forensic dna applications. Likelihood ratio lr tests are provided for the three alternatives to dna identity. Reservations about likelihood ratios and some other aspects. Front matter the evaluation of forensic dna evidence the.
The likelihood ratio is the probability under hypothesis 1 that the suspect profile and the evidencesample profile will both be x, divided by the corresponding. Accurate assessment of the weight of evidence for dna. When quantifying the results of dna analysis, forensic science experts often use the likelihood ratio. Nist urges caution in use of likelihood ratio forensic. The classical likelihood ratio is used routinely for identifying individuals random match probability, kinship paternity index and mixtures combined likelihood ratio. A likelihood ratio compares the probabilities of observing the evidence under two different hypotheses. If the putative father is not excluded by bloodgroup, enzyme, and dna evidence, a paternity index is calculated. Bayesian networks and probabilistic inference in forensic science. Statistical issues the evaluation of forensic dna evidence ncbi. If the ratio is greater than one the evidence e supports hp, with higher values indicating stronger support.
If the ratio is less than one the evidence e supports hd, with smaller values indicating stronger support. They use the sensitivity and specificity of the test to determine whether a test result usefully changes the probability that a condition such as a disease state exists. Evidence based on human deoxyribonucleic acid dna is now widely admitted in court in both civil and criminal cases. Dna evidence bayesian networks and probabilistic inference.
With these new continuous dna interpretation systems and a new, more powerful, dna profiling kit globalfiler there is an opportunity to reexamine the behaviour of a commonly used statistic in forensic science, the likelihood ratio lr. Jan 01, 2017 we have written many times before see the links below about use of the likelihood ratio lr in legal and forensic analysis. Explaining the likelihood ratio in dna mixture interpretation what is the likelihood ratio. If the lr is 1,000, the probability that the profiles are the same is 1,000 times as great if the samples came from the same person as it is if they came from different persons.
Jan 08, 2020 again, as described by the national research council, the likelihood ratio is still one step removed from what a judge or jury truly seeks an estimate of the probability that a suspect was the source of a crime sample, given the observed profile of the dna extracted from samples. Dna testing and its forensic analysis are recognized as the gold standard in. Substantial progress has been made for standard dna profiles, but new issues have arisen in recent years with the advent of more sensitive profiling techniques, allowing profiles to be recovered from minuscule amounts of possibly degraded dna. Sep 03, 2018 now in its second edition, forensic dna evidence interpretation is the most comprehensive resource for dna casework available today. Statistical aspects of interpreting dna profiling in legal.
Oct 17, 2017 many labs are moving toward implementation of the likelihood ratio for interpretation of dna analysis. Statistical issues the evaluation of forensic dna evidence. Likelihood ratios lr are a comparison of two different probabilities to determine which one is more likely. Oct 14, 2010 back to presentations explaining the likelihood ratio in dna mixture interpretation. Intended as a thirdedition companion to the fundamentals of forensic dna typing volume published in 2010 and advanced topics. In theory, the subjective bayesian framework provides a uniquely rational and coherent 4 approach for an. Alan turing, ushered the lr into mainstream scientific thought with his classic book. Forensic dna analysis rests on a firm foundation of biology. The lr is a standard measure of information that summarizes in a single number the data support for a hypothesis.
Should the likelihood ratio be used in the courtroom. The evaluation of weight of evidence for forensic dna profiles has been a subject of controversy since their introduction over 20 years ago. Explaining the likelihood ratio in dna mixture interpretation 4 introduction the likelihood ratio lr appears in many fields of biological, information, physical and social science. The likelihood ratio, then, is a way of summarizing the dna evidence. In this chapter, we consider how to interpret frequencies as probabilities and likelihood ratios and how to make adjustments when a suspect is found through a database search. In a trial, a jury weighs the prosecution hypothesis and the defense hypothesis and determines which is more likely to explain the evidence. In chapter 4, we presented ways to estimate the frequencies of genotypes and profiles in the population. The overall objective of presenting dna and other evidence in court is to help the trier of fact to understand past events. The techniques used in forensic dna analysis are based upon the accepted principles of molecular biology. Dna identification by pedigree likelihood ratio accommodating. Using continuous dna interpretation methods to revisit.
Part i of this paper discusses the issue of what the frequencies associated with dna evidence do and do not mean. Interpreting a major component from a mixed dna profile with an unknown. Though lindley 2014 makes it clear that the lr in bayes formula is the personal lr. Although a considerably large amount of papers and books have been. Jan 17, 2017 kai explains in depth how to calculate likelihood rations using equations and the nomogram method. If you can think of evidence as meaning information that might nudge your decision about some matter in one direction or the other, then the lr is the appropriate numerical summary of the evidence. Synopsis dna testing and its forensic analysis are recognized as the gold standard in forensic identification science methods. How dna evidence creates victims of chance new scientist. Broadly speaking, there are two types of such methods. Identification methods comparing a dna profile from unidentified human remains with that of a direct from the person or indirect for example, from a biological relative reference sample and ranking the pairwise likelihood ratios lr is straightforward and well defined. Perlin, explaining the likelihood ratio in dna mixture interpretation, promegas twenty first international symposium on human identification, san antonio, tx, 14oct2010. In evidence based medicine, likelihood ratios are used for assessing the value of performing a diagnostic test.
Explaining the likelihood ratio in dna mixture interpretation. However, there is a great need for a handson stepbystep guide to teach the forensic dna community how to interpret dna mixtures, how to assign a likelihood ratio, and how to use the subsequent likelihood ratio when reporting interpretation conclusions. This part also addresses issues associated with identifying the hypothesis of interest and characterizing the evidence. Oct 16, 2015 this experiment examined the way that people recruited from amazons mechanical turk n 541 responded to 2 types of forensic evidence a dna comparison and a shoeprint comparison when an expert explained the strength of this evidence 3 different ways. Data from dna test strips are uploaded to a computer and run through an array of probability models until the software spits out a likelihood ratiothe. Jan 28, 2016 the likelihood ratio is simply 1 divided by 2 and provides a simple and compelling measure of probative value of evidence. Interpreting evidence evaluating forensic science in the courtroom.
In the case of dna evidence this ratio is usually large and could run into millions. This interpretation is equivalent to the definition of the socalled bayes factor in bayesian hypothesis testing berger, 1985. Although the process of rendering an opinion regarding the likelihood ratio in regard to the source level usually does not involve enough iterations using empirically derived information not to remain exquisitely sensitive to initial subjective estimates of base rates and other such quantities. Part ii describes an alternate way of presenting dna statistics in court based on bayesian likelihood ratios. The likelihood ratio implies either amplification or attenuation of the prior odds and is as such a measure of evidentiary strength the value of evidence. When it involves a trial the lr is the probability of the prosecutions argument versus the probability of the defenses argument given their starting assumptions. In this hypothetical, let us assume it is reported that, given the dna mixture found in the evidence sample, it is 1,000,000 times more probable that the sample originated from defendant 1 and two unknown unrelated individuals, than three unknown unrelated individuals. Dna testing and its forensic analysis are recognized as the gold standard in forensic identification science methods. In dna identification science, the likelihood ratio lr assesses the evidential support for the identification hypothesis that a suspect contributed their dna to the biological evidence. In such forensic areas the scale of the likelihood ratio may be. Written by leaders in the fields of biology and statistics, including a contribution from peter gill, the father of dna analysis, the book emphasizes the interpretation of test results and provides the necessary formulae in an easily accessible manner. Aug 18, 2010 how dna evidence creates victims of chance.
Dna typing is an important tool in missingperson identification, especially in massfatality disasters. Thus, a likelihood ratio, which is not a probability, can be used to obtain a probability. The ubiquitous transposition fallacy is at work here, as it is in the case law. Over the last two decades, john buckleton 11, ian evett 12, bruce weir and others 14 have brought the likelihood ratio into the. Interpretation builds upon the previous two editions of john butlers internationally acclaimed forensic dna typing textbook with forensic dna analysts as its primary audience. Explaining the likelihood ratio in dna mixture interpretation 5 and nonscientists 10. A practical guide to assigning likelihood ratios will provide a roadmap for labs all over the world and the next generation of analysts who need this foundational understanding. The problem of evaluating this dna evidence in a forensic context, when the suspect is found through a database search, is analysed through a likelihood approach. In dna identification science, the likelihood ratio lr assesses the. This technique enables experts to communicate the weight of the evidence with a single number. Unfortunately, minimizing dna evidence leads to a failure to identify. The problems with dna evidence and testing the atlantic.
1271 123 168 237 399 1211 1046 556 1294 459 182 73 852 1326 750 1465 541 723 309 1411 1320 1494 212 495 661 1145 868 858 860 705 380 385 1259 439 1433 671 870