Circumstantial evidence

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Circumstantial evidence
/ˌsɜːkəmˈstænʃᵊl ˈɛvɪdᵊns/ (n.)
Evidence that changes the prior probability of a fact in issue in legal proceedings without directly attesting to it. A fact that makes a proposition more likely. Thus to be distinguished from direct evidence that addresses the “causal proposition” implied in an allegation of fact directly.

The distinction is important in criminal law. So let us talk about a hypothetical criminal allegation:

Colonel Mustard murdered Reverend Green.

Credibility is a given

In this discussion, for now, let us “control” for witness credibility by assuming all witnesses are speaking the unimpeachable truth.

Direct evidence

Direct evidence may take the form of an eye witness report, from Mrs. Peacock that, say, “Colonel Mustard struck Reverend Green with a lead pipe in the library”. Or recorded video and audio of that act, or the Colonel’s confession that he did so, and so on. These are independent testaments to the action comprising the allegation. If we believe the witness, this evidence will settle the matter: there is no other axis of doubt.

Circumstantial evidence

Circumstantial evidence changes the likelihood the primary allegation is true without addressing it directly: Mrs White’s evidence that she was playing canasta with Colonel Mustard in the drawing room all evening gives the colonel an alibi and makes the primary allegation less likely. A till receipt in Colonel Mustard’s wallet for recently purchased lead piping, on the other hand, makes it more likely. Evidence of a long-standing feud between Mustard and Green makes the allegation more likely.

You might say that direct evidence is evidence which establishes causation while circumstantial evidence establishes correlation. One attests that the event did happen, the other that it is more likely to have happened.

Direct evidence is in a way, a subset of circumstantial evidence in that it resets the probability of the event to 100 percent or zero.

But not all circumstantial evidence is equal. An alibi is powerful. Bloody fingerprints on a length of lead piping found next to the body raises a strong presumption; possession of a till receipt less so. Some kinds of circumstantial evidence is simply consistent with the prosecution case. A hotel guest in possession of lead pipe while the hotel hosts a plumbers’ convention may not help us at all.

The difference between direct and circumstantial: even controlling for credibility, of circumstantial evidence we must still ask a question: how much does this evidence change the probability that the event happened?

An accumulation

With circumstantial evidence there may be major and minor significance — major pieces that get you most of the way to a proving your hypothesis, and minor, supplementary pieces that just confirm and validate it: that “put the matter beyond doubt”.

As such these minor pieces — especially if there are a lot of them — may suffer less scrutiny: they are corroborations; if they turn out to be mistaken they do not disprove anything; they just fail to ice a cake that could well be iced equally well by something else.

So it is, perhaps, with Lucy Letby. All evidence against her was circumstantial. Not just circumstantial but weakly circumstantial — consistent with the prosecution theory rather than tending to exclude of alternatives. This is prime territory for confirmation bias. The “killer fact” — the one piece of accumulated evidence that seems to support the prosecution theory to the exclusion of all others is the notorious nursing rota, on which she was on duty for every suspicious incident.

Are the remaining evidence weaker? Well, imagine the prosecution case without that linking evidence. Say we dispel the idea there was a single perpetrator. No nurse had an unusual confluence if shifts.All of the rest of the evidence — blotched skin, dislodged tubes, air in vessels and so on , was still present, and is presented not to a serial murder trial, but to a series of individual trials against different nurses. Now how seriously would we inspect the chain of custody of samples? The suitability of the insulin test? The value of the “consistency” of the blotching with air embolus? Without this evidence now there is no case to answer. The fact that Nurse Letby was on duty is of no moment because, well, someone had to be on duty, and it being Nurse Letby rather than Nurse Smith or Jones tells us nothing about the incident in question.

The blotching becomes simply “odd blotching” that could mean anything. The insulin poisoning becomes “an unusual insulin reading”. Analysis of insulin tests are vulnerable to base rate neglect after all. Especially tests that are not in the first place suitable for forensic purposes. This is no grounds for a murder prosecution. There remains an obvious reasonable doubt. A probable doubt. This was a premature sick baby who died of natural causes.

All of this changes if there is a highly improbable cluster of incidents and one nurse is a highly improbable common factor in all of them.

Now the blotchiness is not founding evidence but confirmation, with all the implications of that term: if it fits the hypothesis, it adds weight. If it does not it is irrelevant. Only were It to contradict the hypothesis — unlikely — might it have adverse probative value, but that requires a defence advocate to know about it, and be prepared to argue an alternative contradictory hypothesis (which, remember, is not the defence’s responsibility).

So we must come back to the “clinching” evidence — the ward rota — and ask whether it really is the smoking gun it appears to be.

See also

• Direct evidence
• Beyond reasonable doubt
• Prosecutor’s tunnel vision