Royal Statistical Society
12 Errol Street, London EC1Y 8LX, tel: 020 7638 8998
News Release, Tuesday 23 October 2001
Royal Statistical Society concerned by issues raised in Sally Clark case
The Royal Statistical Society today issued a statement, prompted by issues
raised by the Sally Clark case, expressing its concern at the misuse of
statistics in the courts.
"In the recent highly-publicised case of R v. Sally Clark, a medical expert
witness drew on published studies to obtain a figure for the frequency of
sudden infant death syndrome (SIDS, or "cot death") in families having some
of the characteristics of the defendant's family. He went on to square this
figure to obtain a value of 1 in 73 million for the frequency of two cases
of SIDS in such a family.
"This approach is, in general, statistically invalid. It would only be
valid if SIDS cases arose independently within families, an assumption that
would need to be justified empirically. Not only was no such empirical
justification provided in the case, but there are very strong a priori
reasons for supposing that the assumption will be false. There may well be
unknown genetic or environmental factors that predispose families to SIDS,
so that a second case within the family becomes much more likely.
"The well-publicised figure of 1 in 73 million thus has no statistical
basis. Its use cannot reasonably be justified as a "ballpark" figure
because the error involved is likely to be very large, and in one particular
direction. The true frequency of families with two cases of SIDS may be
very much less incriminating than the figure presented to the jury at trial.
"Aside from its invalidity, figures such as the 1 in 73 million are very
easily misinterpreted. Some press reports at the time stated that this was
the chance that the deaths of Sally Clark's two children were accidental.
This (mis-)interpretation is a serious error of logic known as the
Prosecutor's Fallacy. The jury needs to weigh up two competing explanations
for the babies' deaths: SIDS or murder. Two deaths by SIDS or two murders
are each quite unlikely, but one has apparently happened in this case. What
matters is the relative likelihood of the deaths under each explanation, not
just how unlikely they are under one explanation (in this case SIDS,
according to the evidence as presented).
"The Court of Appeal has recognised these dangers (R v. Deen 1993, R v.
Doheny/Adams 1996) in connection with probabilities used for DNA profile
evidence, and has put in place clear guidelines for the presentation of such
evidence. The dangers extend more widely, and there is a real possibility
that without proper guidance, and well-informed presentation, frequency
estimates presented in court could be misinterpreted by the jury in ways
that are very prejudicial to defendants.
"Society does not tolerate doctors making serious clinical errors because it
is widely understood that such errors could mean the difference between life
and death. The case of R v. Sally Clark is one example of a medical expert
witness making a serious statistical error, one which may have had a
profound effect on the outcome of the case.
"Although many scientists have some familiarity with statistical methods,
statistics remains a specialised area. The Society urges the Courts to
ensure that statistical evidence is presented only by appropriately
qualified statistical experts, as would be the case for any other form of
Founded in 1834, the Royal Statistical Society is one of the world's leading
statistical societies, and one of the UK's oldest professional societies.
It awards the professional qualification of Chartered Statistician, or
CStat, to those who meet its stringent criteria for academic achievement and
professional experience in statistics.
For further information contact:
Janine Smedley, Press Officer, tel 020 7639 8998, email firstname.lastname@example.org
Original press release at rss.org.uk