Circumstantial evidence
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Crime & Punishment
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Circumstantial evidence
(n.)
Evidence that changes the prior probability of a fact in issue in legal proceedings without directly attesting to it.”
“Direct” v “circumstantial” evidence
Circumstantial evidence is to be distinguished from direct evidence, which addresses the causal proposition implied in a crime directly.
Let us say the allegation is “JC competed in the Tour de France”. (Let us at once remark what a scurrilous allegation this would be were we not in the realm of the fantastic hypothetical)
“Birgit von Sachsen Rampton saw JC competing in the Tour de France on the television” would be direct evidence of the allegation.
“JC just bought a ludicrously expensive bike, regularly bedecks himself in lycra, talks a lot about Bradley Wiggins and has a one-way Eurostar ticket to Paris for the weekend of 29 June” is — rather weak, it must be said — circumstantial evidence of the allegation.
The distinction is important in criminal law. So let us talk about a hypothetical crime.
Colonel Mustard murdered Reverend Green.
Direct evidence may take the form of an eye witness report from Mrs. Peacock that, say, “Colonel Mustard murdered Reverend Green in the library with a lead pipe”, recorded video and audio of him doing so, Colonel Mustard’s confessions and so on. If we believe the witness, this evidence will settle the matter.
Circumstantial evidence is information, if we accept it, changes the probability of the allegation being true. Colonel Mustard’s alibi that he was playing canasta with Mrs White all evening. A till receipt in his wallet for lead pipe. Evidence of a long-standing feud between Mustard and Green.
You might say that direct evidence is evidence which, if accepted, establishes causation while circumstantial evidence is evidence which, if accepted, establishes correlation. One confirms that the event actually happened, 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 either to 100% or to zero. The only remaining question is “how reliable is the evidence?” Was the witness mistaken? Was she lying? Doubt about reliability may change its weighting to less than 100%
Circumstantial evidence requires the same assessment for reliability, but before even that there is a question: even if completely reliable, how much does the evidence change the probability of the event in the first place?
Colonel Mustard might be a plumber, so habitually in possession of lead piping. He may indeed have bought the lead pipe, but then been robbed of it. Miss Scarlett, Mrs White, Mrs Peacock and Professor Plum may each have had access to lead pipe too. There may have been an unseen intruder with a lead pipe.
An accumulation
With circumstantial evidence there may be major and minor significance — major pieces that get you most of the way to a conviction, and minor supplementary pieces that just confirm and validate: that “put the matter beyond doubt”.
As such these minor pieces — especially if there are a lot of them — might suffer less individual scrutiny: they are corroborations; if they turn out to be mistaken then 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. The “killer fact” was the nursing rota in which Letby was on duty for every suspicious incident.
Imagine the prosecution case without that linking evidence. Say we dispel the idea there was, therefore, a single perpetrator. 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 invidual 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 “odd blotching ". 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.
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.