An increase in baby deaths at the Countess of Chester Hospital’s neonatal unit in 2015 was not steep enough to be considered an “outlier”, the public inquiry over Lucy Letby’s crimes has heard.
Professor Sir David Spiegelhalter, emeritus professor of statistics at the University of Cambridge, told the Thirlwall Inquiry that eight deaths at the unit in that year merited a local investigation but “does not mean necessarily that there is a special cause for it”.
According to hospital data, there was one death at the neonatal unit in 2010, two in 2013 and three in 2011, 2012 and 2014 but the number rose to eight in 2015 and five in the first six months of 2016.
Former neonatal nurse Letby, 35, started work at the Countess of Chester Hospital in January 2012 and went on to murder seven babies between June 2015 and June 2016.
Sir David said the number of deaths between 2010 and 2014 “actually shows surprising consistency” and he would have expected more variability.
Assessing the 2015 figure, he said: “That would generally be considered sufficient to trigger an alert and someone should look at this locally but not extreme enough to be considered an outlier.
“There are about 150 neonatal units in the UK … and therefore we would expect that one signal of this magnitude to occur each year just by chance alone through no underlying cause, nothing special changing at all.
“This is a surprising event within the Countess of Chester but from a national level this is not very surprising at all. We would expect this to happen every year somewhere.”
Sir David added: “The numbers have gone up, they are small numbers but just because numbers have gone up does not mean necessarily that there is a special cause for it, and so it needs to be investigated.”
He said a “real outlier” was “something that you are pretty convinced is not just normal variation but there is some special cause behind it”.
Sir David added that no statistical monitoring system could say why something has happened.
A classic case of an outlier was Bristol Royal Infirmary in the 1980s and 1990s where failings were found in paediatric heart surgery, he said.
Sir David, who headed a team of statisticians for the public inquiry that followed, said: “There was clear water between it and the 11 other centres we were looking at which were reasonably tightly clustered, not completely, and Bristol stuck out very clearly indeed.”
He said that analysis of data from MBRRACE-UK, which collects information on baby deaths in the NHS, showed that in 2015 and 2016 the mortality rate at the Countess of Chester was 10% higher than the average for similar neonatal units with between 2,000 and 4,000 births per year.
The Countess of Chester’s “crude mortality rate” – the number of deaths divided by the number of births – in 2015 was 2.96 per 1,000 which equated to nine deaths, he said.
Sir David said it was the highest in its tier of centres but “only just”, as Blackpool had eight deaths in the same year.
He said: “One would not call that an outlier.”
Sir David added the inquiry into Dr Harold Shipman’s murders showed his mortality rate was extreme but he was “not a clear outlier” as some other GPs had even higher mortality rates “for the very best of reasons”, such as those working with hospices and retirement communities.
Letby, from Hereford, is serving 15 whole-life orders after she was convicted at Manchester Crown Court of murdering seven infants and attempting to murder seven others, with two attempts on one of her victims.
The public inquiry, chaired by Lady Justice Thirlwall, is hearing its final week of evidence at Liverpool Town Hall, with findings expected to be published in the autumn.
