In a high-profile legal case last week, the jury asked the judge to explain what was meant by the term “beyond reasonable doubt”. As is the custom of the English courts, the judge refused to clarify.
But the jurors’ question is legitimate, and the attempt to answer it reveals much, not just about the law, but the analysis of complex problems. English law recognises two principal standards of proof. The criminal test is that a charge must be “beyond reasonable doubt”, while civil cases are decided on “the balance of probabilities”.
The meaning of these terms would seem obvious to anyone trained in basic statistics. Scientists think in terms of confidence intervals – they are inclined to accept a hypothesis if the probability that it is true exceeds 95 per cent. “Beyond reasonable doubt” appears to be a claim that there is a high probability that the hypothesis – the defendant’s guilt – is true. Perhaps criminal conviction requires a higher standard than the scientific norm – 99 per cent or even 99.9 per cent confidence is required to throw you in jail. “On the balance of probabilities” must surely mean that the probability the claim is well founded exceeds 50 per cent.