Letby unit baby death rise 'not extreme' - inquiry

A spike in the number of deaths at the neonatal unit where Lucy Letby murdered babies in 2015 was "not extreme enough" to prompt an external investigation, a public inquiry has heard.

Sir David Spiegelhalter, emeritus professor of statistics at the University of Cambridge, gave evidence about the mortality rate at the Countess of Chester Hospital, where nurse Letby attacked and killed babies.

The Thirlwall Inquiry, set up to examine the circumstances around Letby's crimes, heard that there had been eight deaths in 2015 and five in the first half of 2016. Between 2010 and 2014 there had only been between one and three per year.

Assessing the 2015 figure, Sir David 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."

Letby, now 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.

The nurse, originally from Hereford, has also been convicted of attempting to murder seven other babies.

Sir David told the public inquiry he would expect to see a "signal" - like the 2015 mortality figure for the Countess of Chester - at least once a year in one of the UK's 150 neonatal units "by chance alone".

He added: "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."

The veteran statistician explained that outliers were "something that you are pretty convinced is not just normal variation... there is some special cause behind it".

Sir David pointed out that no statistical monitoring system could reveal the reason or reasons why something had happened.

A classic example of a statistical outlier occurred at Bristol Royal Infirmary in the 1980s and 1990s, he said, which revealed failings in paediatric heart surgery.

Sir David, who headed a team of statisticians for the subsequent public inquiry, said there was "clear water" between Bristol and 11 other centres.

This was not the case for the Countess of Chester Hospital's neonatal unit, he said.

Sir David said its mortality rate in 2015 and 2016 was only 10% higher than the average for neonatal units with similar birth rates.

He said the Countess of Chester was highest in its tier of centres, but "only just".

"One would not call that an outlier," he said.

Letby is serving 15 whole-life sentences 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.

Her findings are expected to be published in the autumn.

Additional reporting by the Press Association.

Listen to the best of BBC Radio Merseyside on BBC Sounds and follow BBC Merseyside on Facebook, X, and Instagram, and watch BBC North West Tonight on BBC iPlayer.