Lawyers and medics are getting a bit of a hammering here. But surely the
statistics profession must share the blame?
The terms 'independent' is used very loosely by statistical people. An
independent variable in regression analysis is one selected the user. The
use of the term independence in the phrase 'statistical independence' is
quite different - and is more worrying.
Statistical independence is actually defined in terms of probabilities. Two
events are defined as independent if the probabilty of one "is the same
whether the other is given or not" (to quote the Oxford Dictionary of
Statistical Terms).
It is easy to see the meaning of statistical independence in the context of
throws of a die. But it is not not clear that this definition has such a
useful meaning in terms of the complexities of statistics about human
activities. Does 'given' mean known, accepted, agreed, taken for granted,
or just assumed ?
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. How did this figure ever
get into print?
Equally puzzlinag to me is the basis for alternative esitmates of
probability. I can go along with the assertion that there is clustering.
But I would really like to know about the assumptions underlying
the calculation of alternative estimates. How do these alternative
estimates get over the problem that such statistics of human activities are
not clearly statistically independent. But surely it is not sound to
multiply probabilities unless statistical independence is assumed?
Ray Thomas
35 Passmore, Tinkers Bridge, Milton Keynes MK6 3DY
Email: [log in to unmask]
Tel/Fax 01908 679081
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