Burden and standard of proof
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The standard of proof is the degree of certainty with which a proposition must be demonstrated: how certain it must be. The burden of proof is the question of who must adduce evidence of the relevant proposition to the relevant standard.
In most cases the burden falls on the prosecution or plaintiff — being the person making the existential claim. But there are circumstances where it may shift to the defendant.
For example, the case of the duck-shooting accident: it can be established beyond reasonable doubt that each of Person A and Person B discharged a gun in the direction of Victim X, believing him to be a duck. Victim X was hit once. It is found, beyond reasonable doubt that one of Person A or Person B was responsible, but as the bullet was not recovered, and there is no evidence as to which of the shooters it was.
A conventional analysis would say that neither shooter can be proved beyond reasonable doubt (or even more likely than not) to have fired the accurate bullet, and that both should be acquitted. I believe the court here may reverse the usual burden of proof and invite each defendant to prove — to what standard it is not clear — it was not her. If neither can, then both will be convicted.
This is rather similar to the idea of joint and several liability, which allows the victim to claim in full against either, and for the implicated defendants to thrash out between them who should pay.