This isn’t quite “a judge has ruled that [Bayes’ theorem] can no longer be used”, but I don’t think it’s good.
The judges decided that using a formula to calculate likelihood isn’t allowed in cases where the numbers plugged into the formula are themselves uncertain (paragraph 86), and using conservative figures apparently doesn’t help.
Paragraph 90 says that it’s already established law that Bayes’ theorem and likelihood ratios “should not be used”, but I think it means “shouldn’t be talked about in front of the jury”.
Paragraph 91 says explicitly that the court wasn’t deciding how (or whether) Bayes’ Theorem and likelihood ratios can be used in cases where the numbers plugged into the formula aren’t themselves very uncertain.
In paragraph 95, the judges decide that (when matching footprints) it’s OK for an expert to stare at the data, come up with a feeling about the strength of the evidence, and express that in words, while it’s not OK for the same expert to do a pencil-and-paper calculation and present the result in similar words.
I think part of the point is that when the expert is cross-examined, the jury will react differently if she says “this evidence is strong because I’ve got lots of experience and it feels strong to me”, rather than “this evidence is strong because I looked up all the frequencies and did the appropriate calculation”.
I do get the impression that the approach of multiplying likelihood ratios is being treated as a controversial scientific process (as if it were, say, a chemical process that purported to detect blood), and one which is already frowned upon. Eg paras 46, 108 iii).
This isn’t quite “a judge has ruled that [Bayes’ theorem] can no longer be used”, but I don’t think it’s good.
The judges decided that using a formula to calculate likelihood isn’t allowed in cases where the numbers plugged into the formula are themselves uncertain (paragraph 86), and using conservative figures apparently doesn’t help.
Paragraph 90 says that it’s already established law that Bayes’ theorem and likelihood ratios “should not be used”, but I think it means “shouldn’t be talked about in front of the jury”.
Paragraph 91 says explicitly that the court wasn’t deciding how (or whether) Bayes’ Theorem and likelihood ratios can be used in cases where the numbers plugged into the formula aren’t themselves very uncertain.
In paragraph 95, the judges decide that (when matching footprints) it’s OK for an expert to stare at the data, come up with a feeling about the strength of the evidence, and express that in words, while it’s not OK for the same expert to do a pencil-and-paper calculation and present the result in similar words.
I think part of the point is that when the expert is cross-examined, the jury will react differently if she says “this evidence is strong because I’ve got lots of experience and it feels strong to me”, rather than “this evidence is strong because I looked up all the frequencies and did the appropriate calculation”.
I do get the impression that the approach of multiplying likelihood ratios is being treated as a controversial scientific process (as if it were, say, a chemical process that purported to detect blood), and one which is already frowned upon. Eg paras 46, 108 iii).