When 'absence of forensic evidence' is not 'neutral'

It is widely accepted that ‘evidence of absence’ (such as an alibi confirming that the defendant was not at the crime scene) is not the same as ‘absence of evidence’ (such as where there is no evidence about whether or not the defendant was at the crime scene).

However, for forensic evidence, there is often confusion about these concepts. If DNA found at the crime scene does not match the defendant is that ‘evidence of absence’ or ‘absence of evidence’? It depends, of course, on the circumstances. If there is a high probability that the DNA found must have come from the person who committed the crime then this is clearly ‘evidence of absence’ - the fact that it does not match the defendant is highly probative in favour of the defence. On the other hand if the only DNA found at the crime scene is actually unrelated to the person who committed the crime, then this is clearly ‘absence of evidence’ – the fact that it does not match the defendant is no more probative for the defence than for the prosecution (so the evidence is ‘neutral’). The problem is that lawyers and forensic scientists often wrongly assume that absence ‘evidence of absence’ is ‘neutral’.

The full report (5 pages) includes a 'proof'  (using a simple Bayesian network model) of how the experts get it wrong in a real example.

Fenton, N. E. (2019). When “absence of forensic evidence” is not “neutral.” https://doi.org/10.13140/RG.2.2.14517.73440

The Bayesian network model is available here. It can be run in the trial version of AgenaRisk