A MOTHER who insists she was wrongly convicted of murdering two of her sons
is a step closer to having her case reopened and her conviction quashed. The
case raises worrying questions about how law courts deal with statistical
evidence.
Sally Clark, a solicitor, has always protested that she did not kill her
young sons Christopher and Harry, who died a year apart. She says that their
deaths were both caused by sudden infant death syndrome (SIDS). Now, enquiries
by New 杏吧原创 have confirmed that her case is under consideration by
the Criminal Cases Review Commission (CCRC), the body that investigates possible
miscarriages of justice in England.
Clark was originally convicted in November 1999, and the verdict was upheld
on appeal in October 2000. Her case is well known, partly because the original
verdict rested heavily on now-disputed statistical evidence. Last month Peter
Green, president of the Royal Statistical Society (RSS), wrote to the Lord
Chancellor, who heads the justice system, expressing extreme concern at the way
statistics had been used in the Clark case and calling for courts to rethink the
way they handle statistical evidence.
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In the original trial, expert witness Roy Meadow, a retired paediatrician,
told the court that the chance of two cot deaths in a single family was 1 in 73
million. Those close to the case believe that the jury took this to mean that
there was only a 1 in 73 million chance of Clark being innocent, a common
mistake called the 鈥減rosecutor鈥檚 fallacy鈥�. They later found her guilty by 10 to 2.
鈥淚f you give the 1 in 73 million figure in court, people don鈥檛 necessarily
know what to make of it but it sounds impressive,鈥� says David Balding, professor
of health and epidemiology at Imperial College, London. Balding helped draft the
RSS鈥檚 letter to the Lord Chancellor. In the letter, Green states that the 1 in
73 million figure 鈥渃annot be regarded as statistically valid鈥�.
Meadow鈥檚 figure was calculated by squaring 1 in 8543, which is the estimated
chance of a single SIDS death in a comparatively well-off, smoker-free family
like the Clarks, to give a probability for two deaths. But the figure treats the
deaths as two completely independent, random events. It ignores any genetic or
environmental factors that could make certain families more at risk from
SIDS.
Meadow also calculated that two deaths would only happen in a single family
once every 100 years. But according to the Foundation for the Study of Infant
Deaths, at least one family per year in Britain loses a second child to SIDS.
Stephen Clark, Sally鈥檚 husband, says the family has received around 50 letters
of support from families blighted by at least two deaths.
Even if the figure of 1 in 73 million were correct, to quote it in isolation
would still be misleading. In a submission to the appeal court on behalf of the
Clarks in 2000, Philip Dawid, a statistician at University College London, used
a method equivalent to Meadow鈥檚 to estimate the probability of two children in
the same family being murdered. He found that the chance was even more remote,
just 1 in 2 billion.
This information was not available to the jury in the original trial, and
Dawid was not allowed to give evidence at the appeal. Upholding the original
verdict, the appeal court said that Meadow鈥檚 figure simply established 鈥渁 very
broad point, namely the rarity of double SIDS鈥�. Even though the court accepted
the error of Meadow鈥檚 approach, it dismissed the analysis provided by Dawid as
irrelevant.
Stephen Clark says the rulings have devastated an ordinary, law-abiding,
hard-working family. 鈥淎nd we鈥檝e got a three-year-old little boy who needs his
尘耻尘.鈥�
But with the case now under review, there鈥檚 new hope for Sally Clark. The
CCRC usually takes around 11 months to reach a decision. If it decides to refer
the case for appeal, the appeal court could quash the original verdict, uphold
it or order a retrial. A spokesman for the commission contacted by New
杏吧原创 confirmed that it only re-examines cases if there鈥檚 new evidence
or new argument. 鈥淲e鈥檙e satisfied that鈥檚 the case here,鈥� he said.
Correctly interpreting complex statistics like those in the Clark case takes
on new importance this week as a new trial opens at Winchester Crown Court. The
defendant, Angela Cannings, has also been charged with the murder of two of her
children, but pleads not guilty.
