At 12:10 AM 10/15/2003 -0700, you wrote:
[EMAIL PROTECTED] (Ivan Balduci) wrote in message
news:<[EMAIL PROTECTED]>...
> Please, may someone explain this fact:
> when the measurements are paired (made on the same subjects), tests for
> equal population SDs based on independent samples cannot be used. Then, a
> test of the equality of the initial and follow-up SDs is equivalent to
> testing for a correlation of 0 between the sums and differences of the
> measurement pairs
>
Cov(X+Y,X-Y) = Cov(X,X) - Cov(X,Y) + Cov(Y,X) - Cov(Y,Y) = Var(X) - Var(Y)
.
.
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