On 29/04/2013 21:24, tim...... wrote:
So according to:

http://www.bbc.co.uk/news/magazine-22310186

"Mathematician" Coralie Colmez claims that if you do a DNA analysis on
an unreliably small piece of evidence and get the same result as the
first time, this strengthens the case that the original result is
correct and uses some coin-tossing explanation to "prove" her case.

I disagree, her corroborating example repeats the same tests on
*different* data whereas doing the DNA test again would be repeating it
on the *same* data (and as such, is completely worthless extra
information).

I think that she has fundamentally missed the point about the
unreliability of the source data here.

Anyone any better ideas than me, what is the right answer here?

(I should really be posting this to ul, but I think the science experts
are in this group?)

TIA

tim

Well, presumably the first test was on a small sample taken from the
knife, and the proposed repeat test would have been on a different
sample. And that, I think, would improve the statistics although without
knowing the exact methodology it's difficult to be sure.

But it was the scientist suggested a repeat test and I am inclined to
think they would have understood the statistics. The journalist's point
was that when courts get the science wrong, people suffer. For example
the Guilford "bombers", the "cot death" lady, the Dutch nurse...