Subtitle: Statistical issues in the investigation of a suspected serial killer nurse Abstract: Investigating a cluster of deaths on a hospital ward is a difficult task for medical investigators, police, and courts. Patients do die in hospitals (the three most common causes of deaths in a hospital are, in order: cancer, heart disease, medical errors). Often such cases come to the attention of police investigators for two reasons: gossip about a particular nurse is circulating just as a couple of unexpected and disturbing events occur. Hospital investigators see a pattern and call in the police. I will discuss two such cases with which I have been intensively involved. The first one is the case of the Dutch nurse Lucia de Berk. Arrested in 2001, convicted by a succession of courts up to the supreme court by 2005, after which a long fight started to get her a re-trial. She was completely exonerated in 2010. The second case is that of the English nurse Lucy Letby. Arrested in 2018, 2019 and 2020 for murders taking place in 2015 and 2016. Her trial started in 2022 and concluded with a “full life” sentence a couple of months ago. There are many similarities between the two cases, but also a couple of disturbing differences. One difference being that Lucy Letby’s lawyers seem to have made no attempt whatsoever to defend her. Another difference is that statistics was used against Lucia de Berk but not, apparently, against Lucy Letby. But appearances are not always what they seem. Report published by Royal Statistical Society on statistical issues in these cases https://rss.org.uk/news-publication/news-publications/2022/section-group-reports/rss-publishes-report-on-dealing-with-uncertainty-i/ News feature in “Science” about myself and my work https://www.science.org/content/article/unlucky-numbers-fighting-murder-convictions-rest-shoddy-stats

1. Richard Gill, SPOR seminar, 10 October 2023
A tale of two Lucy’s
Statistical issues in the investigation of a
suspected serial killer nurse
https://nypost.com/2023/05/25/accused-killer-nurse-lucy-letby-claims-raw-sewage-may-have-contributed-to-infants-deaths/
https://nl.wikipedia.org/wiki/Zaak-Lucia_de_Berk#/media/Bestand:Lucia_post.jpg

2. Trust or Health
Board/unit
Period
Late
fetal
loss
Stillbirth
Early
neonatal
deaths
Late
neonatal
deaths
Post
neonatal
deaths
All
deaths
Countess of Chester
Hospital NHS
Foundation Trust
summary by year
2013
reported
deaths
4 8 3 1 2 18
2014
reported
deaths
2 9 3 1 0 15
2015
reported
deaths
4 12 8 1 0 25
2016
reported
deaths
1 2 7 1 0 11
2017
reported
deaths
0 11 2 2 0 15
2018
reported
deaths
0 6 2 0 0 8
All
reported
deaths
11 48 25 6 2 92
Countess of Chester
4, 4, 9, 8, 4, 2
What set this o
f
(among other things)

3. What convinced the jurors: item 1 (*)
(*) Of course we can’t know. But 3 items convinced media and public

4. What convinced the jurors: item 2

5. Item 2, a reconstruction
Mycroft on 'X'
@ChrisJClarkEsq

6. What convinced the jurors: item 3
Insulin ~ C-peptide ratio, babies F and L
Preparation:
Concentrations can be measured in many di
ff
erent units
1 mole = 6.02214076 × 10^23
molecules.
p (pico) = 10^-12; n (nano) =
10^-9; µ (micro) = 10^-6; m
(milli) = 10^-3
Hence: pmol, nmol, µmol, mmol
1 international unit, 1 “U” or
“IU” or “SU”, was originally
de
fi
ned as the amount of
insulin required to cause
convulsive hypoglycaemia in a
fasted 2kg rabbit
Insulin is a hormone which controls your blood sugar level.
Hypo-, hyper-glycaemia: too little, too much glucose in your
blood

7. Baby F: a test showed that
the insulin concentration
was 4657/pmol/L, the C-
peptide was 169 pmol/L
and the glucose was 1.3
mmol/L.
Within two hours of the
blood test the baby’s
blood sugar level was ~4
mmol/L, which is normal
Baby L: similar http://pathlabs.rlbuht.nhs.uk/insulin.pdf
http://pathlabs.rlbuht.nhs.uk/seafrm.htm )
What convinced the jurors:
item 3
Insulin ~ C-peptide ratio,
babies F and L

8. • 2015, 2016: con
fl
icts at CoCH between management, medical
consultants, and nurses
• Everyone had noticed alarming rise in death rate in NICU
• Incidents seemed often to happen in Lucy Letby’s shifts
• The RCPCH was charged with an investigation
• Late 2016, con
fi
dential report submitted
• Early 2017, redacted report made public
• May 2017, Hospital asks Cheshire Constabulary to investigate
cause of deaths
Obviously, not. The trial lasted 10 months
Was that all?

9. • May 2017, police announce investigation, newspapers report
• June 2017, retired paediatrician Dewi Evans turns up at Cheshire police
station o
ff
ering his services
• Police give him medical notes of about 32 incidents, about half of which
were deaths, the other half collapses + successful resuscitation
• Many deaths and collapses concern same infants and many concern
members of twins, triplets; many were very premature
• 2018, Lucy Letby taken in for questioning and then released
• 2019, Lucy Letby taken in for questioning and then released
• 2020, Lucy Letby arrested [TV vans in the street before police arrive]
Obviously, not. The trial lasted 10 months
Was that all?

10. • 32 charges, of which 8 murders and the rest attempted murder
• October 2022, trial starts
• June 2023, trial ends, jury retires to consider verdict
• July and August: jury returns verdicts; judge announces sentence
• Verdict: 7 murders and 15 murder attempts
• Sentence: full life
Obviously, not. The trial lasted 10 months
Was that all?

11. • A mountain of evidence was brought in by the prosecution
• All of it is circumstantial
• Lucy was never seen to do anything wrong
• Every event has a natural explanation
• The medical evidence says babies F and L were not administered
a large unauthorised amount of insulin
• The rise in number of deaths has a natural explanation
Obviously, not. The trial lasted 10 months
Was that all?

12. • Police believe hundreds more babies have been murdered or
harmed and a new investigation is proceeding, studying medical
records of 1000 more incidents
• Prosecution has appealed against not guilty verdict for baby K
• Trial scheduled for July 2024. Reporting restrictions have been
imposed
• A public enquiry will take place to
fi
nd out why the hospital directors
tried to stop the medical consultants from going to the police
• Lucy has asked for an appeal (she has a right to ask, but it may not
be granted). Three judges will decide
The prosecution and the public want more
What next?

13. • The 32 incidents were compiled by the senior medical consultants (“the gang of 4”)
• They include all deaths from Jan 2015 to June 2016
• This is exactly the period that Lucy Letby worked on the ward
• They include all collapses in the same period when Lucy was believed to be on duty
• Approx equal numbers of each
• Dewi Evans found something odd in most events which happened when Lucy was there
and in each case he fantasised a murderous attack which might have caused that anomaly
• He did not spot the anomalous insulin results so did not have babies F or L in his list. Dr
Breary found them in 2018.
• Doctors Breary, Gibbs, Jayaram, and NN (CoCH), and Evans (retired) seem all to have lied.
There is evidence of connections between the gang of four and Evans.
Reconstruction by an amateur (Mark Mayes)
What happened in 2017?

14. • The defence did not
fi
nd any experts
• Neither prosecution nor defence used statistical or
epidemiological evidence
• The single defence witness was the plumber
• The defence agreed that the insulin evidence meant there was a
murderer on the ward
• The great British public has never been so uneducated
The defence; the plumber; the tabloids; social media;
the jury system; changes to the rules of the last 20 years
How could it go so wrong?

15. • A few brave souls are starting to speak out
• UK media labels them as crackpots and conspiracy theorists
• At least four quali
fi
ed scientists have “pro Lucy” websites
arguing the trial was unfair
• Something will happen (CPS appeal / Lucy appeal / Enquiry)
• I think that the tide is turning
Who knows
What will happen next?

16. • Lucia: management and doctors worked together; Lucy: they were
fi
ghting
• Adversarial versus inquisitorial system
• Culture, time (20 yrs apart), media, social media
• Lucia’s defence did their best to defend her. Lucy’s seemed not too
• In both cases, failures of top medical doctors plus failures of management
caused unnecessary loss of lives, a notable nurse got the blame
• UK: political issues around NHS; systematic NHS failings
• UK: social inequality; poor education
• UK: systematic erosion of right to a fair trial over several decennia
My answer: yes and no. The same but far worse.
And is either case only about statistics, or only about medicine, or
something more complex?
Is Lucy’s case essentially the same as Lucia’s?

17. • Overdispersion
• Public misunderstanding and distrust of statistical science
• Misunderstanding and distrust within science
• The magical power of numbers
• Lies, damned lies, and statistics
• Police investigate crimes. Doctors cure patients. Psychiatrists
associate criminal behaviour with psychiatric disorders.
• In the hospital situation, people die “normally” all the time.
See the report of the Royal Statistical Society!
Appendix: issues for SPOR
https://rss.org.uk/news-publication/news-publications/2022/section-group-reports/rss-publishes-report-on-dealing-with-uncertainty-i/