R v Adams
Encyclopedia
R v Adams [1996] 2 Cr App R 467, [1996] Crim LR 898, CA
and R v Adams [1998] 1 Cr App R 377, The Times, 3 November 1997, CA, are rulings that ousted explicit Bayesian statistics
from the reason
ing admissible before a jury
in DNA
cases.
ed and an identity parade was arranged. The woman failed to pick him out, and on being asked if he fitted her description replied in the negative. She had described a man in his twenties and when asked how old Adams looked, she replied about forty. Adams was 37; he had an alibi
for the night in question, his girlfriend saying he had spent the night with her. The DNA was the only incriminating evidence heard by the jury, as all the other evidence pointed towards innocence.
left at the scene. The defence
argued that the match probability figure put forward by the prosecution (1 in 200 million) was incorrect, and that a figure of 1 in 20 million, or perhaps even 1 in 2 million, was more appropriate. The issue of how the jury should resolve the conflicting evidence was addressed by the defence by a formal statistical method. The jury was instructed in the use of Bayes's theorem by Professor Peter Donnelly
of Oxford University. The judge told the jury they could use Bayes's theorem if they wished. Adams was convicted and the case went to appeal. The Appeal Court judges noted that the original trial judge did not direct the jury as to what to do if they did not wish to use Bayes' theorem and ordered a retrial.
At the retrial the defence team again wanted to instruct the new jury in the use of Bayes's theorem (though Prof. Donnelly had doubts about the practicality of the approach). The judge asked that the statistical experts from both sides work together to produce a workable method of implementing Bayes' theorem for use in a courtroom, should the jury wish to use it. A questionnaire
was produced which asked a series of questions such as:
These questions were intended to allow the Bayes factor
s of the various pieces of evidence to be assessed. The questionnaires had boxes where jurors could put their assessments and a formula to enable them to produce the overall odds
of guilt
or innocence
. Adams was convicted once again and again an appeal was made to the Court of Appeal. The appeal was unsuccessful and the Appeal Court ruling was highly critical of the appropriateness of Bayes's theorem in the courtroom.
Actually, it is possible that these guidelines are seriously wrong. The other matching people are probably relatives of the accused (possibly unknown as relatives) and may well live nearby. This may change the significance of the DNA evidence considerably.
Court of Appeal of England and Wales
The Court of Appeal of England and Wales is the second most senior court in the English legal system, with only the Supreme Court of the United Kingdom above it...
and R v Adams [1998] 1 Cr App R 377, The Times, 3 November 1997, CA, are rulings that ousted explicit Bayesian statistics
Bayesian statistics
Bayesian statistics is that subset of the entire field of statistics in which the evidence about the true state of the world is expressed in terms of degrees of belief or, more specifically, Bayesian probabilities...
from the reason
Reason
Reason is a term that refers to the capacity human beings have to make sense of things, to establish and verify facts, and to change or justify practices, institutions, and beliefs. It is closely associated with such characteristically human activities as philosophy, science, language, ...
ing admissible before a jury
Jury
A jury is a sworn body of people convened to render an impartial verdict officially submitted to them by a court, or to set a penalty or judgment. Modern juries tend to be found in courts to ascertain the guilt, or lack thereof, in a crime. In Anglophone jurisdictions, the verdict may be guilty,...
in DNA
DNA
Deoxyribonucleic acid is a nucleic acid that contains the genetic instructions used in the development and functioning of all known living organisms . The DNA segments that carry this genetic information are called genes, but other DNA sequences have structural purposes, or are involved in...
cases.
Facts
A rape victim described her attacker as in his twenties. A suspect, Denis Adams, was arrestArrest
An arrest is the act of depriving a person of his or her liberty usually in relation to the purported investigation and prevention of crime and presenting into the criminal justice system or harm to oneself or others...
ed and an identity parade was arranged. The woman failed to pick him out, and on being asked if he fitted her description replied in the negative. She had described a man in his twenties and when asked how old Adams looked, she replied about forty. Adams was 37; he had an alibi
Alibi
Alibi is a 1929 American crime film directed by Roland West. The screenplay was written by West and C. Gardner Sullivan, who adapted the 1927 Broadway stage play, Nightstick, written by Elaine Sterne Carrington, J.C...
for the night in question, his girlfriend saying he had spent the night with her. The DNA was the only incriminating evidence heard by the jury, as all the other evidence pointed towards innocence.
Use of Bayesian analysis in the court
The DNA profile of the suspect fitted that of evidenceEvidence (law)
The law of evidence encompasses the rules and legal principles that govern the proof of facts in a legal proceeding. These rules determine what evidence can be considered by the trier of fact in reaching its decision and, sometimes, the weight that may be given to that evidence...
left at the scene. The defence
Defense (legal)
In civil proceedings and criminal prosecutions under the common law, a defendant may raise a defense in an attempt to avoid criminal or civil liability...
argued that the match probability figure put forward by the prosecution (1 in 200 million) was incorrect, and that a figure of 1 in 20 million, or perhaps even 1 in 2 million, was more appropriate. The issue of how the jury should resolve the conflicting evidence was addressed by the defence by a formal statistical method. The jury was instructed in the use of Bayes's theorem by Professor Peter Donnelly
Peter Donnelly
Peter Donnelly, FRS is an Australian mathematician and Professor of Statistical Science at the University of Oxford. He is a specialist in applied probability and has made contributions to coalescent theory...
of Oxford University. The judge told the jury they could use Bayes's theorem if they wished. Adams was convicted and the case went to appeal. The Appeal Court judges noted that the original trial judge did not direct the jury as to what to do if they did not wish to use Bayes' theorem and ordered a retrial.
At the retrial the defence team again wanted to instruct the new jury in the use of Bayes's theorem (though Prof. Donnelly had doubts about the practicality of the approach). The judge asked that the statistical experts from both sides work together to produce a workable method of implementing Bayes' theorem for use in a courtroom, should the jury wish to use it. A questionnaire
Questionnaire
A questionnaire is a research instrument consisting of a series of questions and other prompts for the purpose of gathering information from respondents. Although they are often designed for statistical analysis of the responses, this is not always the case...
was produced which asked a series of questions such as:
- "If he were the attacker, what's the chance that she would say he looked nothing like the attacker?"
- "If he wasn't the attacker what's the chance that she would say he looked nothing like the attacker?"
These questions were intended to allow the Bayes factor
Bayes factor
In statistics, the use of Bayes factors is a Bayesian alternative to classical hypothesis testing. Bayesian model comparison is a method of model selection based on Bayes factors.-Definition:...
s of the various pieces of evidence to be assessed. The questionnaires had boxes where jurors could put their assessments and a formula to enable them to produce the overall odds
Odds
The odds in favor of an event or a proposition are expressed as the ratio of a pair of integers, which is the ratio of the probability that an event will happen to the probability that it will not happen...
of guilt
Guilt
Guilt is the state of being responsible for the commission of an offense. It is also a cognitive or an emotional experience that occurs when a person realizes or believes—accurately or not—that he or she has violated a moral standard, and bears significant responsibility for that...
or innocence
Innocence
Innocence is a term used to indicate a lack of guilt, with respect to any kind of crime, sin, or wrongdoing. In a legal context, innocence refers to the lack of legal guilt of an individual, with respect to a crime.-Symbolism:...
. Adams was convicted once again and again an appeal was made to the Court of Appeal. The appeal was unsuccessful and the Appeal Court ruling was highly critical of the appropriateness of Bayes's theorem in the courtroom.
Statistical analysis of DNA
The only evidence against Adams was the DNA evidence. His age was substantially different from that reported by the victim, the victim did not identify him and he had an alibi which was never disproved. The 1 in 200 million match probability calculation did not allow for the fact that the perpetrator might be a close relative of the defendant - an important point, since the defendant had a half-brother in his 20s whose DNA was never tested.Court guidelines for using statistical evidence in DNA cases
The Court of Appeal subsequently produced guidelines for the way match probabilities should be explained to jurors. Judges should say something along the lines of the following.- '"Suppose the match probability is 1 in 20 million. That means that in Britain (population about 60 million) there will be on average about 2 or 3 people, and certainly no more than 6 or 7, whose DNA matches that found at the crime scene, in addition to the accused. Now your job, as a member of the jury, is to decide on the basis of the other evidence, whether or not you are satisfied that it is the person on trial who is guilty, rather than one of the few other people with matching DNA. We don't know anything about the other matching people. They are likely to be distributed all across the country and may have been nowhere near the crime scene at the time of the crime. Others may be ruled out as being the wrong sex or the wrong age group."'
Actually, it is possible that these guidelines are seriously wrong. The other matching people are probably relatives of the accused (possibly unknown as relatives) and may well live nearby. This may change the significance of the DNA evidence considerably.
External links
- Text of first Court of Appeal Judgement on BAILII.
- Text of second Court of Appeal Judgement on BAILII.
- "Appealing Statistics", SignificanceSignificance (journal)Significance, established in 2004, is a magazine published quarterly by both the Royal Statistical Society and the American Statistical Association. The major part of the content consists of articles on topics of statistical interest presented at a level suited to a general audience. It is not a...
, 2(1):46-48, 2005 March.