At 23:00 24/07/05 +0100, Ray Thomas wrote (in small part):
>Lawyers and medics are getting a bit of a hammering here. But surely the
>statistics profession must share the blame?
>[snip]
>The general point to be made is that it is actually difficult to find
>meaninful statistice of human activities that are truly independent of each
>other. Statistics of human activities are socially or organisationally
>constructed not 'given'. Poetically put 'No man is an island'.
>
>Meadow's mutliplication to get a probability of one in 73 million assertion
>was obvously wrong. Two deaths in the same family cannot be considered as
>statistically independent of each other because they occured in the same
>family. But I don't recall the RSS or statisticians jumping up and down
>when this figure of 73 millions was first used.
As many have said, it's rather amazing that the defence apparently did not
see fit to obtain any statistical expert opinion, which could well have
resulted in the figure (hence quite probably the whole prosecution case)
being challenged.
However, having a defence witness who incorrectly quantifies the
probability by the use calculations based on unreasonable assumptions is
merely an added twist to the (essentially non-statistical) problem of the
underlying 'unreasonable assumption' which has been played out in court (to
the detriment of the defendant) many times before.
There have been other highly publicised cases in which, without any
statistical input or attempted quantification of probabilities, parents
have been convicted of harming/killing children - because (without
attempted quantification) it was considered 'beyond reasonable doubt' that
a similar extremely rare fate could befall two or more children in the same
family from 'natural causes'. We have heard of cases in which rare
inherited diseases have resulted in two or more siblings being abnormally
susceptible to breaking bones, brain damage, bruising etc. One (US) case I
have seen repeatedly on TV ('Medical Detectives', on Discovery Channel, I
think) was of a pair of siblings who, it turned out, both had a very rare
genetic disorder which produced effects which simulated poisoning. ... and
there have been many more.
In all these cases, the underlying error has really been on the part of
clinicians (and lawyers) who have failed to acknowledge the possibility of
common genetic (or environmental) factors which might dramatically increase
the chances of two incredibly rare events occurring in the same family. In
other words, the clinicians who express such views are effectively
asserting that there is no (clinical) way in which the events could be
non-independent. If they briefed a statistician to estimate probabilities,
they presumably would again make that same assertion - and unless the
statistician were able to successfully challenge that 'clinical judgement',
then I suppose the statistician would have little choice but to undertake
the calculation on the basis of that asserted assumption.
I certainly agree that it's a great pity that statisticians (individually
or corporately) did not manage to somehow intervene in this case - but it
seems to me that it's fairly clear that the underlying failings DID lie
with the medics and lawyers.
A more philosophical/sociological question relates to the issue of this
sort of 'probabilistic conviction', even though one has to accept that the
great majority of convictions (at least of contested prosecutions) are
necessarily to some degree 'probabilitsic'. What if the chances of this
being a 'natural phenomenon' really HAD truly been 73 million to 1? Do we
think that the courts understand the nature of random processes - such that
even an incredibly improbable event is equally likely to occur to Family A
'tomorrow' as to any other family in 500 years time? A similar mindset
could easily result in winners of the Lottery (or roulette, or whatever)
being automatically jailed, on the grounds that it was 'vanishingly
improbable' that any given individual would win without cheating!
Kind Regards,
John
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