Lucy Letby: commonality

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Show me a man who believes in conspiracy theories and I’ll show you someone who has never organised a surprise party.

—Lee Harvey Oswald.

Ordinarily when a defendant appears on 22 counts of murder and attempted murder there is a wealth of compelling direct evidence: eyewitnesses, fingerprints, incriminating forensics, and related corroborations that point not just to murder, but to defendant specifically committing each one.

Ms Letby’s case is different in that regard. The evidence, which we might describe as a “wealth”, but only with some irony, largely concerns whether in each collapse a crime has been committed at all. There is not much addressing who might have done it. The hours of baffling expert evidence about venous air emboli, immunoassays, c-peptide levels and so on were there to rebut the presumption that these collapses were just the ordinary run of medical emergencies one would expect in a busy intensive care unit.

The prosecution seems tacitly to be founded on and assumption that if malicious origin can be established it, goes without saying the perpetrator was Ms. Letby.

This article addresses that tasset assumption and looks at legal reasoning that might justify it.

Prelude: tragedies in the air

On October 29, 2018, a Boeing 737 operated by Lion Air, a private carrier operating primarily in Southeast Asia, crashed into the Java Sea, killing all on board.

Five months later Ethiopian Airlines, a large national carrier operating mainly in Africa, lost a 737 MAX when it crashed into farmland near Bishoftu.

At around the same time, there were a series of other air accidents: an Air Niugini Boeing 737 crashed into a lagoon in Micronesia. A Fly Jamaica Boeing 757 overran a runway in Guyana in November 2018. A Saha Airlines Boeing 707 crashed in Iran in January 2019.

The crashes seemed, as air crashes generally are, unrelated: they happened to different airlines, on different continents, at different altitudes, over different terrain, at different times of the year and in different weather conditions.

Air crash investigators were called in and began to review the black box flight data records.

They soon began to notice unusual similarities between the two 737-MAX crashes.

MCAs: a villain in the room

Sometimes, when you look closely, the circumstances surrounding apparently unrelated events are so similar as to invite the inference that they have a common cause. Airliner crashes are vanishingly rare and, except where planes crash into each other, are almost always unrelated.

They tend to arise from a complex mix of causes: pilot error, terrorism, atmospheric conditions, garbled air traffic control instructions, freight loading, autopilot mis-programming, mechanical malfunctions and even ground-to-air missile attacks. Each tends to be sui generis because the global aviation system learns from its mistakes. It is very good at isolating, minimising and not repeating catastrophic risks.

The two 737-MAX crashes stood out. They were uncannily similar. Both airliners were more or less brand new. Both crews experienced erratic altitude changes shortly after takeoff. In both cases, the computerised navigation system repeatedly, inexplicably, forced the plane’s nose down while the crew struggled to force it back to level. In both cases, the autopilot eventually overrode the crew’s manual intervention, whereupon the planes plunged irrecoverably towards the earth.

Air crash investigators soon identified a common culprit: Boeing’s state-of-the-art “Manoeuvring Characteristics Augmentation System” (MCAS) was installed on both airliners.

“MCAS” was designed to automatically push the plane’s nose down in certain conditions, to prevent stalling. But it relied on readings from a single “angle-of-attack” sensor, with no redundancy if it failed. When the sensor provided incorrect data, as it did in both cases, MCAS repeatedly forced the nose down. Boeing had not alerted airlines about the new system, and pilots were not trained how to manage it. The system could, in any case, override pilot inputs and there was no easy way to disable it in an emergency.

There was a villain in the room: MCAS.

A villain in the room at Countess of Chester?

With that analogy in mind, we come to the Countess of Chester Hospital in 2016. It had suffered, in effect, a series of baffling air crashes over a short period. But what had caused them? Was this just a random cluster, or was there a common cause? Was there a villain in the room?

Like airline accidents, individual hospital collapses tend to be independent: each patient arrives with her own pathology and can expect a health outcome consistent with hospital treatment for that pathology. Hospitals are meant to make people better. Generally when people die in hospital it is because the hospital’s treatment of their condition did not work — rather than because something else altogether did them in.

If environmental factors at the hospital are creating collapses we should expect an improbable convergence of symptoms, conditions and outcomes to indicate iib in the same way we would with air crashes. We should expect corroborating evidence, in other words, of the conditions causing the collapses.

A word about the standard of proof

Aquick reminder that the Crown must make all the running in a criminal trial. Proving guilt is the thoroughly, at times tediously, analytical job of satisfying the jury as to every element of every component of every crime on the indictment.

The Crown does this by presenting evidence of all of those components. A big part of the judge’s instructions to the jury will be to guide them through the complicated syllogisms they must navigate to find each guilty verdict.

The exercise is in no sense “impressionistic”. To reach a guilty verdict, the jury must ask itself, for each component of each offence, whether it is satisfied that the evidence presented at trial, taken in the round, proves that component. If the jury is not so satisfied about any individual component, the judge will tell them, “You must acquit the defendant of that charge”. Some leave some grisly calling cards — a bullet-holed queen of hearts on the forehead, say — unique enough to raise the inference that, if the “Lonely Hearts Killer” did that one, then he must have done these ones, too. So the law has developed some techniques that address the difficulty of producing evidence against a person determined to hide it without lowering the standard of proof.

One of these is the “strikingly similar” evidence rule: a jury may infer a defendant’s guilt of multiple crimes even when there is compelling evidence for only some of them, if there is a “signature” to the offending. If the crimes, or how they were committed, are so unusual and distinctive that they function as a “behavioural fingerprint”.

The courts will not infer such a thing lightly: a “signature” needs to be strong enough to support the inference, that, since the defendant did this one, the only plausible explanation is that she did those ones, too. If it is, this is a convenient heuristic for police: it saves them having to find eyewitnesses for all the bodies stacked up in the county morgue that are decorated with bullet-holed playing cards.

But the signature must be highly unlikely to occur by chance or to be replicated independently by a different perpetrator.

There is very little direct evidence implicating Ms. Letby for the charged offences. If the Crown wishes to claim she has nevertheless left behind a “behavioural footprint”, it has two problems.

Non-similarity

There was pre-meditation, calculation and cunning in your actions. [...] You used a number of different ways to try to kill them, thereby misleading clinicians into believing the collapses had, or might have had, a natural cause or were a consequence of a developing medical condition.

—Mr. Justice Goss’s sentencing remarks, 21 August 2023.

Firstly, the collapses aren’t strikingly similar. To the contrary, if things were as Mr. Justice Goss described them when sentencing her, Ms. Letby was only artful in how she switched up her methods to make the offences look different. The Court cited this very calculatedness as a measure of her cruelty.

According to the Crown, Ms. Letby attempted murder, variously, by injecting air into babies’ bloodstreams, stomachs, administering factitious insulin, overfeeding, tampering with breathing tubes, disabling oxygen saturation alarms and, in one case, by physically attacking a premature infant. But it led no evidence of her actually doing any of these things.

Evidence or not, this seems to be a long way from “striking similarity”. There is no specific “signature” to distinguish these events from accidental medical emergencies. The presumption that Ms. Letby might just have been cunning enough to avoid leaving evidence does not change the Crown’s burden, unless it can bring evidence of that acuity.

It does not seem to have done that.

The Crown must, therefore, find cogent evidence to tie Ms. Letby, specifically, to every allegation: if not direct eyewitness evidence of her actions, then cogent circumstantial evidence that it can only have been her.

It does not seem to have done that, either.

The Crown did present evidence that the collapses were consistent with someone’s deliberate harm — the insulin smoking gun, for example — but very little of it pointed specifically to Ms. Letby doing something to harm the children. As we will see, the closest the Crown got, in the case of Child K, is some evidence of Ms. Letby not doing something.

Non-identification

Secondly, the striking thing about the Crown’s evidence against Ms. Letby specifically is, but for the staff attendance chart — and we will come to that — how weak it was. Were it not for the “commonality” angle, there was almost no evidence against Ms. Letby at all.

We can see this best by taking the commonality out of the equation. How good is the evidence for each charge in isolation?

If this charge were laid against Ms. Letby by itself, would there be enough evidence to find Ms. Letby guilty of murder beyond reasonable doubt?

For most of the charges, the answer is plainly, “No”.

Virtual red hands

The most promising individual charge is the notorious episode where, according to the Crown, Dr. Jayaram caught Ms. Letby “virtually red-handed”.

If it is to be believed — a big if, but let’s say, for now — Dr. Jayaram’s testimony was the most compelling direct evidence there was against Ms. Letby. Mr. Johnson KC opened the Crown’s case in the retrial as follows:

“[Another nurse ] had left [Child K] connected to a ventilator that was breathing for her, and [Child K] was connected to another machine that was checking her heart rate and saturations.

“If either the heart rate or the oxygen saturations in the blood fall below a pre-determined level, then alarms will sound, and if there was an issue, those alarms would have sounded. But they did not because somebody had disabled them.

So, when Dr. Jayaram walked into the nursery, he saw Lucy Letby was standing over [Child K] and her blood oxygen levels were falling, but the alarm was not sounding. Not only that, but Lucy Letby was doing nothing.

We say that, in those circumstances, the only reasonable thing for a nurse to do would be to call for help or use [equipment] to breathe for the child.”

—Nick Johnson, KC, opening in the retrial of Child K, 12 June 2024.

The Crown’s case was that Ms. Letby attempted to suffocate Child K by dislodging his breathing tube and switching off his desaturation alarm. The infant was, therefore, only saved by Dr. Jayaram’s timely intervention. But no one saw Ms. Letby dislodging the breathing tube or disabling the alarm. Others recalled it sounding. Nor was there evidence it had been disabled or interfered with. Premature infants are tiny, move around a lot, and often dislodge their own breathing tubes.

The Crown’s clinching evidence was Dr. Jayaram’s. He told the court he entered the room to see Ms. Letby standing over Child K’s incubator “doing nothing” while the infant collapsed.

Though the Crown presented this as the smoking gun, it is a long way from that.

It is possible Dr. Jayaram misconstrued Ms Letby’s observation, assessment, or waiting for assistance as “inaction”. Nor does he explain what alerted him to the unfolding scenario. Perhaps the desaturation alarm did go off, as others remember hearing it. Perhaps someone did call for help. Perhaps that person was Ms. Letby. We need not be detained by details: beyond emphasising that there was a great deal of doubt what happened at the time, and it has been further clouded by the years that have passed since, it does not much matter.

Still, Mr. Johnson alluded to a kind of signature when he framed his case to the jury:

“The reason [Child K] was desaturating was because that the breathing tube had been displaced and we suggest that the fact Lucy Letby was doing nothing and the fact the alarms were not sounding is evidence from which you can conclude that it was Lucy Letby, the convicted murderer, who had displaced the tube.

We say Lucy Letby had been caught virtually red-handed by Dr. Jayaram.”

There is only one reason for that parenthetical about Ms Letby’s convictions: to pull them in as a kind of signature. To hope they can buttress the weak evidence for the immediate case.

To say, “there was a killer on the wards, so —”

Another way of saying “virtually red-handed” is “not red-handed”, after all.

But what are the odds—

This doesn’t sound like it should work, but there is one way it still might: if the probabilities bear it out. Now it might aggravate that part of the commentariat that insists this case has nothing to do with statistics, but bear with me. Where there is a dearth of direct evidence and, for all Mr. Johnson’s protestations, the “signature” isn’t strong enough to tie unwitnessed evidence to confirmed criminal behaviour, perhaps the simple improbability of Ms. Letby being present for every collapse might tell us something.

Ms. Letby’s account is that this was by coincidence.

There were 22 incidents spread over 15 months. In that period, there were approximately 1,300 8-hour shifts. What is the chance a full-time nurse would present at all 22 events by sheer coincidence?

We can work it out. Even with overtime, having factored in weekends and holidays, a full-time nurse would work no more than a third of all shifts. Her chance of being present for any single event is, therefore, one in three.

To get probabilities for a series of events you multiply their probabilities, so the chance of her being at all 22 events by coincidence is one third multiplied by itself, 21 times.

That is, there is a one in 31 billion probability Ms Letby was at 22 randomly selected events by chance.

For the less mathematically minded, that is no chance at all.

The “red-handed” episode was the strongest piece of identification evidence.

We know this because, whenever a post-mortem was conducted at all, the outcomes were “unascertained” or “natural causes”. Of course, this does not disprove that the children were murdered, but nor is Ms Letby required to prove her innocence: the Crown is required to prove murder. This is compelling prima facie evidence that there is reasonable doubt. The Crown has the burden of dispelling that doubt.

In most cases, individually, it would get nowhere near.On this evidence the crown would struggle prosecution faces significant challenges proving defendant’s intention, or that she took concrete steps toward committing murder beyond being merely present, or ruling out all reasonable alternative explanations for the desaturation.

Plainly, as a single charge this would not be enough to charge Ms. Letby let alone convict her. Her unusual behaviour only becomes more suspicious because there are other unexplained collapses, and she is in the frame for those too.

But this is — on its face — the most compelling pieces of evidence against her. It is the only forensic evidence that directly associates Ms Letby, personally with a collapse.

So we have a kind of odd circularity here, where a series of weak inferences, all of which would fail individually, somehow create enough solidity to succeed together.

But the collapses were not like that.

This is especially savvy in an environment like an intensive care unit, where patients are prone to collapse anyway. In another case of absent evidence being treated as evidence of something, we are asked to take from this lack of similarity how good it means she was, how devious. This missing commonality passed instead as proof of wickedness. This is a deft sleight of hand.

Entertain, for a moment, that innocent explanation. Notwithstanding the consultants’ darkest suspicions, these clumped episodes of hypoglycaemia, collapse, distress and infection really were unrelated.

What relevance would a staff attendance chart be then?

For, thanks to that chart, the Crown gave the jury the clear impression that these 22 events necessarily had a common, human cause, and there was no other human who fitted it.

The crown appears to have made no serious attempt to establish it.

A conspiracy of one

“Did you know that, for a disguise, elephants paint the soles of their feet yellow, and hide upside-down in custard?”

“Nonsense.”
“Have you ever seen an elephant hiding upside-down in custard?”
“No —”
“SHOWS WHAT A GOOD DISGUISE IT IS.”

—JC’s favourite elephant joke.

This is a conspiracy theory, of sorts — albeit a conspiracy of one.

This is not such a silly idea. A conspiracy theory is a narrative for disputing a common-sense view of the world. It asserts that a given commonsense view is false, and has been engineered by those with the motive and capacity to do so in order to obscure the true, sinister, state of the world.

Only the scale of the proposed deception implies a secret confederacy: to carry off most conspiracy theories, a criminal mastermind requires help.

It is the nature of this one that Ms. Letby worked alone. In all other respects, it meets all the criteria of a conspiracy theory. A total lack of confederates ought to make the plan less plausible, but unfalsifiability is another feature of conspiracy theories, and it works here to address that problem: the very lack of evidence of conspiracy is, itself, evidence of conspiracy. Evidence tending to contradict the conspiracy is planted, as part of the conspiracy. Confirmation bias is weaponised.

All the multivariate infinity of causes operating on the world — all the flapping butterflies, all the piled-up grains of sands, all of sod’s laws, all unintended consequences and all system effects that contribute to produce unexpected calamity — all of these commonplaces and lead-footed familiars we must reject, however well acquainted we are with them, in favour of a single devious, calculating murderess, ghosting unobserved around a densely populated ward, systematically — but also somehow opportunistically — knocking off infants without leaving a trace while bystanding colleagues saw nothing and could do nothing.

Base rate neglect

Based on this learnt distribution, for each of the years (2015, 2016) where there was a claimed ‘spike’ in neonatal deaths, we found that the probability of observing at least the number of deaths that were observed at CoCH by chance alone was not small (34.2% probability for the 9 deaths in 2015, and 40.4% probability for the 8 deaths in 2016). There was, thus, nothing especially unusual about the ‘spike’ in deaths at CoCH in 2015 and 2016.

How unusual was the spike in neonatal deaths when Lucy Letby was working? Scott McLachlan and Norman Fenton, September 2024

This obliges us to consider another case of base rate neglect. There have been many in this case. We have two possibilities: an unusual cluster of collapses in a neonatal ICU, and a serial murderer.

All else being equal, the odds favour the former, heavily. Clusters can and do happen: what we have encountered we would expect by chance somewhere in the UK each year. Serial murderers, of any kind, let alone in a healthcare context, are vanishingly rare.

That is the baseline: these are prior probabilities. We should then look for validating “posterior” evidence that can make one or other of these mutually exclusive alternatives more likely.

Corroborating evidence, that is to say. We find little to corroborate the serial murderer hypothesis, and it tends to fall apart on examination. Ms. Letby’s constant presence turns out not to be a wildly improbable event , but a criteria for selecting the events. The consultant who, after a a couple of years of silence, suddenly recalled catching her “virtually red-handed” standing over the incubator of a desaturating infant is confronted with his own contemporaneous note, never handed to police, in which we recorded that Ms Letby in fact summoned him to help.

Mean time there is no “calling card”. There is no telltale “signature”. The collapses do not bear the hallmarks of an improbable coincidence that suggest they have a common origin.

To explain this the judge infers — without any evidence — that Ms Letby is a calculating master criminal. But if so there should be some corroboration of this. Some history of cruelty, or spitefulness; some anecdotes that speak to a sharp, unscrupulous, depraved mind.

But there seems to be none.

Some flail around throwing out favourite folk diagnoses: Munchausen’ syndrome by proxy. But there is no evidence of that either.

If Lucy Letby is a cunning monster, she is a singular one. She is uncunning in her internet searches, her habit of squirrelling away handover notes and handing over private notes made after counselling sessions.

But despite this carelessness in covering her tracks, she leaves no meaningful corroborating evidence . Is this evidence of meta-cunning? So cunning she plants “careless”, somewhat hinky, but nonetheless non-incriminating evidence to throw us further off the trail?

Even if she is, It is hard to get away with this! We should expect her to slip up. We should expect plenty of arrows to point definitively at her. Even a calculating dervish must leave some consequence unconsidered. Something must show up.

What we have is a statistical unlikelihood — put into evidence without supporting statistics — the CPS shut down a live line of enquiry that might have provided it — hinky huh — that, but for this one I evidenced presupposition, makes absolutely no sense whatsoever.

The conspiracy theory angle. Also covers this: in the ordinary course, when a certain unusual event happens, you would expect around it there to be some consequences, follower ones, knock on effects, and observable repercussions of that event and stop these, they would normally be evidence. This sort of thing corroborates primary event that you are talking about.

In any situation where you have a markedly unusual event but none of the surrounding repercussions, ramifications, secondary causes, ripple and so on Colin you need some explanation for their absence. A conspiracy is a convenient one because the conspiratorial nature of the main event itself is the motivation grounds justification and explanation for the lack of corroborating evidence.

Notes

Disclosure rules — what the crown must show. It is asymmetrical. Defence doesn't. Privilege (as set out)

The effect of separate charges and the importance of commonality... If there were just one charge of murder, and the Crown’s evidence were standing next to one baby isn't cogent evidence of murder.

See also

• Lucy Letby
• Conspiracy theory
• Base rate neglect