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England and Wales Court of Appeal (Criminal Division) Decisions
You are here: BAILII >> Databases >> England and Wales Court of Appeal (Criminal Division) Decisions >> Adams, R. v  EWCA Crim 2474 (16 October 1997)
Cite as:  1 Cr App Rep 377,  Crim LR 316,  1 WLR 1500,  EWCA Crim 2474,  1 Cr App R 377,  3 All ER 805,  1 Cr App R 377
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B e f o r e :
(Lord Bingham of Cornhill)
MR JUSTICE POTTS
MR JUSTICE BUTTERFIELD
|R E G I N A|
|- v -|
|DENIS JOHN ADAMS|
Smith Bernal, 180 Fleet Street, London EC4
Telephone 0171-421 4040
(Official Shorthand Writers to the Court)
MR ORLANDO POWNALL and MISS SUSAN TAPPING appeared on behalf of
Crown Copyright ©
THE LORD CHIEF JUSTICE:
"That is an outline of the defence case and if this were any other case that would be virtually all I had to say to you before asking you to retire and consider your verdict, but of course this case is not like any other case and, as I said at the beginning, it is unique, I think; unique because it is the first case in which a jury have been asked to operate Bayes Theorem and that is because it is, I believe, the first case in which the prosecution have relied almost entirely upon the DNA evidence; evidence which they say is positively overwhelming and the Crown have, as they nearly always do in cases involving DNA, put their case into figures and indeed you have heard those figures. The chance of some other man in the white European population of this country having the same matching bands as were found in this case is one in 200 million, and the lower figures if the position is different.
Before you came into this case I was asked by the defence whether I would allow them to put their case into figures and it seemed to me, to cut a long story short, only fair that if I were to allow one side to do so, I should allow the other to also, and the particular way in which they have sought to do that is by the use of Bayes Theorem. That is a result in probability theory, a rule of logic, the point of it being that once certain numerical judgments are made, certain others must necessarily follow. So it is that you have with you a list of questions, lettered almost through, straight through the alphabet from A to X inclusive, in answer to which you may put either a number or a percentage number, as the case may be, and if you were to do it, having done it, you will have reached the last page on which is a formula and, having substituted figures for the letters and, with the help of your calculators, done the multiplying and dividing, you would come to the resulting equation and if you then follow the three steps, as instructed, you will have an end figure and that would be your view of the probability that this defendant was the rapist. In my view it would not be right for me to tell you more about the questionnaire because if I did I would simply be trespassing on what is essentially your task. You have the questions and if you think you can answer them you have the little boxes to put the answers in. I dare say none of you have ever used Bayes Theorem to decide anything in your lives before, nor have you ever given numbers or percentages to your views in logical steps. It is entirely a matter for you whether you use this method to reach your verdict or whether you use the methods which juries in this country have used for many, many years, pretty satisfactorily. It is, perhaps, the difference between what Mr Lambert called the statistical approach and the common sense approach.
I ought to remind you, though, that although Professor Donnelly is the prime advocate for the use of this theorem and, indeed, takes the view that it is the only way of doing it logically, Mr Lambert agrees that Bayes is a logical and consistent way of expressing in figures the non-DNA evidence, but he also thinks that people without statistical training and experience will find it a very difficult and complicated exercise. Although he is not against the exercise, Mr Lambert has very serious misgivings about putting it into practice in a jury trial because, for example, it does not cover all the relevant factors or all the relevant evidence, or all that you might think was relevant. It might be thought too that the questionnaire does not include a box for you to enter your figure for Mr Adams's own evidence and how he gave it, or the difficulties or otherwise that Miss [M] had in being asked to identify her rapist. Those are just examples and there may be others.
If you feel able to use the questionnaire to operate Bayes Theorem and you find it almost as easy as kiss your hand to give the answers, then there you have the opportunity to do it, having not only your own copies but you will have when you go out an extra blank one to fill in your collective view if you want to. If you do not wish to use it that is your privilege and your own private decision and no one will criticise you for not using it. There is absolutely no compulsion on you to use it at all. It is there if you want to use it and follow the instructions given. It was suggested by Mr Thwaites that you might think it only fair to this defendant for at least one of you to do it. I hope he will forgive me if I discourage that and for this reason: Your duty, when it comes first thing tomorrow morning, is to retire, consider your verdict amongst yourselves, all of you together and not with one huddled in a corner with his calculator. All of you should take part in your discussions, each listening to the arguments of the others and in the end reaching your collective verdict, the verdict of you all. If you want to use it then please use it in a collective way so that you are all having an input into it and putting the answers, if you feel able to, in the blank copy that you will have first thing tomorrow morning."
"It has been suggested that it may be appropriate for the statistician to expound to the jury a statistical approach to evaluating the likelihood that the defendant left the crime stain, using a formula which gives a numerical probability weighting to other pieces of evidence which bear on that question. This approach uses what is known as the Bayes Theorem. In the case of Adams (Denis)  2 Cr App R 467 this Court deprecated this exercise in these terms at p482:
"It has been suggested that it may be appropriate for the statistician to expound to the jury a statistical approach to evaluating the likelihood that the defendant left the crime stain, using a formula which gives a numerical probability weighting to other pieces of evidence which bear on that question. This approach uses what is known as the Bayes Theorem. In the case of Adams (Denis)  2 Cr App R 467 this Court deprecated this exercise in these terms at p482:'To introduce Bayes Theorem, or any similar method, into a criminal trial plunges the jury into inappropriate and unnecessary realms of theory and complexity deflecting them from their proper task.'
We would strongly endorse that comment."
We note that the judgment given in this case on 26 April 1996 has been the subject of consideration in  Crim LR 898, where the court's observations find favour with the commentator.