3.1 Legal decision-making
This takes us neatly to the next issue to explore: the relationship between a ‘finding’ by a forensic scientist, and the decision of the court. Put bluntly: if a forensic scientist were to state in evidence that there is a probability of 1 in 1 million that a DNA sample found at a scene of crime matches the DNA profile of an accused, then would the court be bound to accept his finding?
One case where forensic evidence was crucial to the prosecution is known as R v Adams  (R in this case stands for Regina and is used where the Crown Prosecution Service brings a case to court). Use the Wikipedia entry for R v Adams  [Tip: hold Ctrl and click a link to open it in a new tab. (Hide tip)] case as a starting point for research to answer the question:
What position does the court take regarding Adams and the statistical value of evidence?
As with all Wikipedia entries, you should only use the article as an initial jumping-off point and check what is being said against other sources.
The position the court takes is quite clear. In the appeal case, Adams was accused of rape but the only evidence against him was DNA. Later Court of Appeal guidelines said that the judge’s directions to the jury about the statistical value of the evidence in effect usurped their role.
The Court of Appeal said that the jury should have been told:
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.
You can read the short article mentioned on the Wikipedia page: Donnelly, P. (2005) ‘Appealing statistics’, Significance, vol. 2, no. 1, pp. 46–8 to see the view of Professor Donnelly, who took part in the appeal.
If you have access to a legal database, you can find analysis of the R v Adams case in a system such as Westlaw, which is an authoritative source of case reports. Once in the database select Cases from the menu and enter R v Adams in the Party Names box and search. You should see a result from 1996 and can select the Case Analysis to read.