Sir David Spiegelhalter urges caution and humility when interpreting mortality data, inquiry told

The “unusual” spike in baby deaths on the neonatal unit where Lucy Letby worked had a 0.008% probability of occurring given its previous mortality rate, a public inquiry has been told.

Sir David Spiegelhalter, the emeritus professor of statistics at the University of Cambridge, said the sudden rise in deaths in 2015 was “surprising” and enough to warrant an internal investigation.

However, he added that from a national perspective the increase was “not very surprising at all” and was “not extreme enough to be considered an outlier”.

The Thirlwall inquiry has heard that three babies died on the Countess of Chester’s neonatal unit in 2012, followed by two in 2013 and three in 2014.

This rose to eight in 2015 and there were a further five deaths in the first six months of 2016. Letby was removed from the neonatal unit in July 2016.

The former nurse, now 34, is serving 15 whole-life prison terms after being convicted of murdering seven babies and attempting to murder seven others between June 2015 and June 2016. She maintains her innocence.

The use of statistics in Letby’s first trial has been one of the central causes of concern raised by experts who believe she is the victim of a miscarriage of justice.