Letby Canards, #2: “the defence should have led expert evidence”

The next in our series of mini-posts looking at specific arguments about the Lucy Letby case

Air embolism was reported as early as the 19th century, in both the pediatric and adult surgical practice. The nonspecific nature of the signs and symptoms of vascular air embolism as well as the difficulty in documenting the diagnosis does not allow the true incidence of it to be known.

—Stephanie Gordy & Susan Rowell, Vascular air embolism, International Journal of Critical Illness and Injury Science, cited in the National Library of Medicine

Diagnosing an air embolism with modern technology and adequately conducted post-mortems is relatively straightforward.

Diagnosing one half a decade later is impossible using post-mortem X-rays and histology slides.

—Michael McConville , X, 10 February 2025

The more we learn about Dr Evans’ expert evidence the clearer it becomes why Ms. Letby’s defence didn’t call expert evidence to contradict it. The legal team are barred by obligations of confidentiality and legal privilege from explaining themselves so, in atonement for my misguided earlier criticism of their tactics, here is my best guess at why Ms. Letby did not lead any expert evidence. In essence, because the prosecution evidence was speculative, and thanks to the vagaries of criminal procedure, rebuttal evidence would not have helped.

As Dr. McConville notes, it is possible to make a clear diagnosis of air embolus, but you need to do it at the point of death and with specific tests. Dr. Evans did not have this opportunity: he first offered his services to the CPS in May 2017. By the time he was reviewing case files, more than two and a half years had passed since the first reported incident.

Symptoms are, at the best of times, non-specific and diagnosis is therefore difficult. Dr. Evans had only weak, two-year-old evidence to ground his opinion. He was acting more like an archaeologist than a doctor.

And here our old friend probability comes in. Medical diagnosis is a process of probabilistic inference of causes from observed symptoms. The weaker your data, the higher your chance of drawing incorrect inferences. This is a truism: the less you have to go on, and the more time has passed, the more scope there must be for degradation, contamination or outright loss of important evidence, and the intervention of cognitive bias whether or not you believe there was foul play.

Cross-examination versus contradictory evidence

Now: the defence has two principal means of undermining the credibility of weak prosecution expert evidence: it can cross-examine it, or it can introduce its own contradictory expert evidence. Or both.

Cross-examination

Cross-examination is a magical art. A skilled practitioner can use it to prove right is left, up is down and O.J. Simpson was framed.

Barristers are experts in this art. They have the advantage over witnesses because, by asking leading questions, they can force a witness to reframe an issue in a way it might not like. Far from being somehow underhand or unfair, leading witnesses in this way is a core skill of adversarial cross-examination.

When examining Dr. Evans, Ms. Letby’s defence had much better material to work with than did F. Lee Bailey. They could, and evidently did, put to Dr. Evans that his diagnosis was highly speculative, that it was supported by little hard evidence, that he was not very experienced in air emboli, had long since retired, was suffering confirmation bias and so on.

Dr. Evans could, and did, deny those defence assertions, but risked seeming defensive, flustered, or in denial in the face of a well-constructed cross-examination.

Where a witness is “over his skis”, cross-examination has the advantage of being incisive and devastating, but, unless the witness accepts defence propositions, it is not, of itself, evidence. When, months later, the judge sums up for the jury, he will not include a review of the cross-examination, however devastating it was, unless the witness accepted it.

The defence may remind the jury of cross-examination in its closing arguments, but these do not have the force of impartiality that the judge’s summary of evidence does.

Defence experts

Secondly, the defence might undermine a prosecution expert by presenting contrary expert evidence.

Famously, Ms. Letby’s defence did not, and has been widely criticised for that, including by me. But, I now think, wrongly. Remember the defence’s theory of the case: the evidence for air embolus is flimsy: too weak, too stale, and the conclusions any qualified expert could draw would be too speculative to be of probative value.

That being so, just as the prosecution evidence was weak, so would defence evidence be weak. A credible defence expert would be obliged to start by averring that it was not possible to say what happened.

“Look, I don’t think it is air embolus but, honestly, it is impossible to tell, because the data is so sketchy.”

This is hardly a knockout punch, even before you get to cross-examination. And get to cross-examination it surely will, where it presents the prosecution with an open goal:

“You don’t know, do you? On your own theory, you don’t have enough evidence to judge. For all you know, it could be air embolus, couldn’t it?”

A credible witness would be obliged to agree. It could be air embolus. While the possibility of air embolus gets nowhere near the required burden of proof — that it could not reasonably be anything else — that concession would form part of the defence expert’s evidence. It would be summarised by the judge at the end of the trial. As they retired, the jury would be left with the impression that the Crown expert was certain it was air embolus, whereas the defence expert agreed you couldn’t rule air embolus out.

Even though their expert ought to be pouring cold water on the case, the procedural idiosyncrasies of British criminal justice conspire to do the opposite.

Therefore, the defence had no real choice but to pin their hopes on demolishing Dr. Evans’ credibility with cross-examination. A defence expert would not much help, and might well make things worse.

The problem was not a failure to lead defence evidence, but the Court’s decision to admit flakey prosecution expert evidence in the first place. The defence strenuously objected to this, appealing to the Court of Appeal, but was roundly rejected, largely on the grounds that it has been open to the defence to lead evidence and it chose not to.

Expert evidence regime

The issue here is not the defence tactics, but the expert evidence regime itself. Experts are an exception to the usual rule that witnesses may not offer their opinions or draw inferences. Experts may draw inferences, on scientific and technical matters, where an ordinary jury would lack the expertise to form competent conclusions alone. The experts are called as impartial advisors to assist the court and help the jury.¹

Despite that, under the present regime they are retained and remunerated by parties to the litigation. That introduces a plain conflict of interest. It is understood to be resolved by the adversarial system: each side can test the other’s evidence by cross-examination.

But there is a tension between the premise on one hand that juries need help to assimilate and understand technical evidence and on the other that they can effectively arbitrate between experts about that technical evidence when those experts cannot even agree with each other.

Over a decade ago, the Law Commission recognised the potential for injustice when using experts. In recommending an overhaul of the entire expert evidence regime it observed:²

Too much expert opinion evidence is admitted without adequate scrutiny because no clear test is being applied to determine whether the evidence is sufficiently reliable to be admitted.

This problem is exacerbated in two ways:

First, because expert evidence (particularly scientific evidence) will often be technical and complex, jurors will understandably lack the experience to be able to assess the reliability of such evidence.

There is a danger that they may simply defer to the opinion of the specialist who has been called to provide expert evidence.

Secondly, in the absence of a clear legal test to ensure the reliability of expert evidence, advocates do not always cross-examine experts effectively to reveal potential flaws in the experts’ methodology, data and reasoning.

Juries may therefore be reaching conclusions on the basis of unreliable evidence. This conclusion is confirmed by a number of miscarriages of justice in recent years.

The Law Commission’s recommendations have not (as yet) been enacted, though the Letby case has reawakened some enthusiasm for them.

It is worth backing up to remember why normal witnesses are not allowed to offer opinions or draw inferences. People can be tested on what they saw or heard, but their interpretations may be coloured by bias, speculation, or faulty reasoning in ways that are much harder to identify or test.

Expert witnesses are an exception to this rule for two reasons: firstly, the technical nature of the evidence means there is a real danger of uninformed inference without their assistance; and secondly, the technical matters are sufficiently settled that the correct inferences to be drawn from the evidence are not wildly controversial. In other words, with experts there ought to be little danger of inferences coloured by bias, speculation, or faulty reasoning.

If there is a significant difference in expert opinion, either the science is not sufficiently settled or the evidence to hand is not sufficiently clear for the facts to be established.³

Air emboli are rare. Literature about them is scant. What there is suggests, at the best of times, their symptoms are “non-specific”. That means they are not unique to any particular cause.⁴

The evidence Dr. Evans reviewed was two years old and limited in nature. His theory was necessarily speculative. While air embolus could not be ruled out, there was at least a material possibility the observed symptoms were caused by something else. While its evidence was not before the court and has not been tested by prosecution cross-examination, the recently convened panel of experts — chaired, remember, by the very specialist whose paper Dr. Evans cited — at least suggests Dr. Evans’s theory was controversial. Bear in mind, the burden of proof, on the prosecution, requires not just an arguable theory, but proof beyond reasonable doubt.

The question becomes whether introducing controversial speculations as “expert evidence”, with the impression that the “expert” label provides of “settled, sure, official facts” is excessively prejudicial given the evidence’s likely probative value.⁵

The Court of Appeal dismissed Ms. Letby’s appeal against the admissibility of Dr. Evans’ evidence, in part because it favoured the adversarial approach to evidence reconciliation, and in part because the defence elected not to lead contrary evidence, despite having it. The Court was satisfied that the jury was able to assess Dr Evans’ credibility in light of his cross-examination.

This reflects the present approach to expert evidence and glosses over the Law Commission’s concerns. By criticising its tactical (and apparently reasonable) decision not to call its own expert witness, this puts the defence in a cleft stick: symptoms being as non-specific as they are the evidence cannot support a clear finding of air embolus one way or another, so few experts will be prepared to prove a negative, and those who are will not be good experts.

Experts have their exemption from the rule against opinions so they can make technical but professionally uncontroversial inferences. Experts should not disagree about these things. A “battle of experts” is no way of resolving technical evidence the jury cannot otherwise understand. The Law Commission is right about this.

A scenario where opposing expert witnesses are at odds with each other indicates the subject matter is not, for one reason or another, suitable for authoritative resolution by technical experts. It implies there is, Q.E.D., reasonable doubt.

That, surely, is where the matter should have rested.

1. See: CPS Expert Evidence guidance.

2. See the Law Commission report of 22 March 2011, which recommended that expert opinion evidence should not be admitted unless it was adjudged to be sufficiently reliable.

3. There is a third alternative: at least one of the experts is not, in fact, sufficiently versed in the subject matter to offer a sensible opinion of it. By way of professional courtesy, I presume that is not the case.

4. In medical terminology, “non-specific” symptoms are signs of illness or health issues that could be associated with many different conditions, making it difficult to pinpoint a single diagnosis based on those symptoms alone.

5. Under Section 126(2) of the Criminal Justice Act 2003 the Court retains its common law powers to exclude evidence that would be unfair or whose prejudicial effect would be likely to outweigh its probative value.