"The Sally Clark case was not one equation gone wrong. Misstated recurrence risk, conditional-probability confusion, and undisclosed microbiological evidence all mattered."
What Happened
Sally Clark, an English solicitor, lost her sons Christopher in 1996 and Harry in 1998. She was convicted in 1999 of murdering both children and sentenced to life imprisonment. Her convictions were quashed in 2003 after a second appeal.
The 1-in-73-Million Figure
At trial, pediatrician Roy Meadow estimated that two sudden infant deaths in a family like Clark's had probability about 1 in 73 million. The calculation squared a single-death estimate of roughly 1 in 8,543, treating the events as independent. The Royal Statistical Society later objected that this independence assumption was not justified and that the number was misleading.
Even a valid P(evidence | innocent) is not P(innocent | evidence). Moving between them requires explicit competing hypotheses, priors, and the probability of the evidence under each. One cannot repair the case by inserting a single guessed prior and announcing a posterior probability of guilt.
Other Evidence Mattered
The successful second appeal also involved microbiological results relating to Harry that had not been disclosed to the defense. The Court of Appeal held that the conviction was unsafe. Statistical error was important, but it should not be presented as the only cause of the conviction or the appeal's only ground.
Clark was released after more than three years in prison. She died in 2007 from acute alcohol intoxication. The case became a central warning about expert statistical testimony, disclosure, dependence, and conditional probabilities in criminal proceedings.