Because the defense must first provide some evidence that a mistake was, or could have been made, in order for the jury to receive such an instruction.
> Because the defense must first provide some evidence that a mistake was, or could have been made ...
No, they only need to describe the statistical probability of an error -- they don't need to uncover specific evidence that an error was made in a particular case. And they can empanel expert witnesses to describe the kinds of errors that are typical of the method, but again, without having to uncover evidence unique to the present case.
> Presumably that's beyond the reach of a defendant on legal aid.
Yep. Some might call that a defect in our legal system (that fair representation is out of reach of indigent defendants). Others, cynics all, might assert that if you can't pay for the justice, don't do the crime.
> No, they only need to describe the statistical probability of an error
I can see that comprehension is not your strong suit. As I said, they must show that a mistake was, or could have been made.
You can show that a mistake could have been made by showing the statistical probability of an error. Generally, this statistical evidence must be sufficiently specific as to apply to the circumstances of the matter at hand.
> I can see that comprehension is not your strong suit.
Here is what you said: "Because the defense must first provide some evidence that a mistake was, or could have been made"
It's false, I corrected you. A statistical probability of a false positive is not evidence that a mistake was, or could have been, made. It is a statistical prediction for a particular outcome, and it's not a mistake -- the practitioner is assumed to be performing the procedures as specified, and is blameless.
If I say that, given the usual distribution of population IQs (mean 100, SD 15), that 4.78% of the population will have an IQ at or above 125, is that a mistake? Will someone be brought up on charges of negligence?
Actually, it is. Contrary to your claim, a statistical outlier, a false positive, is not called a "mistake" and no one is brought up on charges. The legal term is typically "Act of God" or another similar term.
> And in all fairness, when dealing with the law, what is "legal" trumps what is "logical."
