One of the most infamous figures in modern criminal history: Lucy Letby, a former neonatal nurse who was found guilty of killing seven babies and attempting to murder seven others in her care at the Countess of Chester Hospital in England. Her trial and subsequent conviction have raised questions about the safety of the judicial process and the reliability of evidence presented in this high-profile murder case.
Born in 1990, Letby was working in a neonatal unit when, from 2015 to 2016, she began to die, with an alarming number of infant deaths occurring under mysterious circumstances. Prosecutors maintain she did the deed, and the court found her guilty, but many questions remain and are explored in the lengthy narrative summaries.
The trial presented critical evidence in the form of a staffing rota, which indicated that Letby was working every time a suspicious death or collapse occurred. For many who followed the case, the rota became the prosecution's lighting bolt. But this evidence is now under serious threat. Statisticians, in particular, are concerned that it has been misused and misleadingly presented to the jury. Peter Green, a professor of statistics, has said that he and others are not at all comfortable with the way this evidence was interpreted. Indeed, he has laid out for us a number of reasons for concluding that this form of evidence just does not add up.
Some critics assert that the staffing rota may have been overinterpreted. The jury that convicted her earlier this month seemed to have no trouble understanding the case, finding its way through what were at times knotty questions. They solved the riddles the prosecution set before them and came up with a unanimous answer.
A staffing rota showed she had been on duty for each suspicious collapse or death between June 2015 and June 2016. The rota was a pricipal part of the case as an important graphic of the case against her. A number of statisticians have publicly questioned its usefulness. Notably, Peter Green, a professor of statistics and a past President of the Royal Statistical Society has commented.
"The chart appears to be very convincing, but there are a number of issues with it," he said.
"A big thing is that it only describes 25 of the bad events which happened in this period.
It doesn’t include any of the events that happened when Lucy was not on duty."
There were at least six other deaths and numerous collapses. Professor Green said the chart also does not reflect the fact that Letby was working more shifts.
"It’s a natural human thing. We all see patterns that are not there," he said.
"The danger is that this evidence can be very compelling to the non-professional, and over interpreted."
E.G.Price
https://en.wikipedia.org/wiki/Royal_Statistical_Society
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