Lucy Letby and the problem with statistics

First Fred West, now Lucy Letby. At this rate, it won’t be long before Herefordshire has produced more serial killers than it has miles of dual carriageway. You might assume growing up in one of England’s loveliest counties would make people placid, but then you haven’t spent half your life stuck behind a caravan on the A465. They may not all kill people, but Herefordshire people overtake like psychopaths.

But I’m going to park my Monmouthshire prejudice here and suggest that something about the Lucy Letby conviction seems off to me. I’m not going to talk about the medical aspects, because I’m not qualified to comment. Instead I’m going to talk about the statistical aspects, where I’m not qualified to comment either, but then neither is almost anyone else. The interpretation of statistics, especially those involving probability, seems to present an extreme case of what is known as the Dunning–Kruger effect, where a person’s confidence in their own ability in any field is inversely correlated with their true level of competence.