Lucy Letby: confirmation bias

Take the following hypothetical scene, which describes Lucy Letby’s experience at the Countess of Chester Hospital:

Internal investigation

A hospital experiences a cluster of deaths and collapses materially in excess of its usual rates for such events.

Staff notice a particular nurse was present during an abnormal proportion of the collapses.

Concerned at the possibility of foul play, hospital management investigates the collapses, focusing on those where the nurse was present. Those where she was absent are removed from the cluster.

The investigation is widened to include other unexplained collapses at which the nurse was present. Some are added to suspicious cluster.

External investigation

Though hospital management has not thoroughly considered alternative possibilities — that the cluster was simply “statistical noise” or that it had a different, non-malicious explanation — it presents its suspicions to the police.

Police investigate the narrow question “whether there are sufficient grounds to prosecute the nurse” and not, specifically, “what else could have caused the original cluster?” Police do not interrogate the hospital’s internal investigation methodology.

Instead, Police seek firstly evidence consistent with foul play — typically technical medical analysis — and secondly, given there has been foul play, evidence consistent with the nurse being behind it — this will be weak circumstantial evidence of “unusual behaviour” consistent with guilt. These two steps are markedly distinct. Notably, Police disregard behaviours “consistent with” the nurse’s innocence.

Media involvement

The media pick up the story. Through repetition and hyperbole, the nurse’s identified “guilty“ behaviours are sensationalised to be ever more incriminating.

“Experts” speculate on the nurse’s motivation: “God complex”, “craving attention”, “Munchausen’s by proxy”, secretive behaviour” for example. Though they frequently contradict and none were formally diagnosed before (or after) the investigation, these descriptions are taken in the round as further corroboration of the nurse’s guilt.

This may sound like a carefully tailored account of Ms. Letby’s apprehension and conviction, but it is not: it describes the pattern followed in the prosecutions of Beverley Allitt, Benjamin Geen, Colin Norris, Susan Nelles, Jane Bolding, Victorino Chua, Lucia de Berk, Daniela Poggiali and dozens of other health professionals prosecuted in the western world for murder by poisoning.

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First find a suspect, then find a crime

Now, this is not to imply that none of these individuals committed murder nor, necessarily, that all were wrongly prosecuted — though some of them were — but rather to note that the “healthcare serial murder” scenario presents distinctive challenges uncommon in normal crime scene investigations.

Normally, criminal investigations start with little doubt there’s been a crime, but a lot about who committed it. A dead body with an ice-pick in its ear and a bullet-holed playing card in its breast pocket is all but certain to have been murdered. The investigator’s main challenge is working out by whom: once identified, linking the villain to the crime tends to be straightforward.

With healthcare serial murder cases, the “route to suspicion” is curiously backward. It starts with a cluster of events that could be but, all else being equal, are most likely not, criminal in nature. Assuming, nevertheless, they are criminal, investigators then hypothesise about the most likely suspect. Inevitably, their suspicion falls upon the person present at the most identified events.

There may then follow an exercise in “refining” the cluster to better fit the “foul play” theory. Collapses at which the nurse was not present may be dropped — recharacterised as non-suspicious — while previously unsuspicious collapses during the nurse’s shifts may be upgraded to “suspicious” in light of her presence.

The refined cluster will, by her constant presence alone, much more stron