David Jones wrote:
>
> Robert J. MacG. Dawson wrote:
> > KR wrote:
> >>
> >> Hi,
> >>
> >> If I have two different surveys from which I calculate values of P1
> >> and P2, would Cov[P1, P2]=0? The two surveys were performed in
> >> different years and have different sample sizes.
> >
> > You cannot have a covariance without a pairing between the elements
> > of the two sets of observations; covariance is a measure of whether
> > bigger-than-average measurements in one set tend to be paired with
> > bigger-than-average measurements in the other.
> >
> > You cannot have a pairing with different sample sizes.
>
> While the context is totally unclear, the immediately above is not
> right (incomplete pairing is enough to lead to correlation)... if the
> second sample were an expanded version of the second, with the people
> (items?) in the first carried over to the second, then there would be
> enough pairing to lead to correlation.
The covariance would not be between the two samples, but
between the two subsets of samples for which the pairing existed.
-Robert Dawson
.
.
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