Joanne,
Thank-you very much.
I will take your advice and only allow for clustering if this increases the
SEs.
I can see that it is possible for units within clusters to be more different
than units sampled randomly from the population as a whole. But I think the
opposite is more likely to be true (at least for the clusters I have).
I found references to the intra-cluster correlation (or intra-class)
correlation saying that estimates can be negative.
But rather confusingly, they usually also say that the total variance can be
divided into a between cluster component and a within cluster component, and
that the intra-cluster correlation is var(b)/[var(b)+var(w))]
Some references even give this as the definition of the intra-cluster
correlation.
Accordingly most calculations are based on an ANOVA table and only lead to
positive values.
Tom
Joanne Garrett wrote:
> Negative values
> imply that people within a cluster (or PSU) are more different from each
> other than they would have been had you sampled randomly (i.e., not by
> cluster). Intuitively this makes no sense.
>
As the great diagnostician Charcot was fond of saying, '�a ne l'emp�che
pas d'exister'. There is one occasion on which this may indeed be true:
where the PSU is a group of people who form a team or social unit
characerised by a division of labour. A family, for example, or a
community mental health team, will combine people with different
specialisations and points of view. Two people randomly sampled from
within such a PSU will be more sharply differentiated than two people
sampled from the population.
Certainly, I recally that my mother and I were capable of producing a
kappa of close to minus one in matters of musical taste.
--
Ronan M Conroy ([email protected])
Senior Lecturer in Biostatistics
Royal College of Surgeons
Dublin 2, Ireland
+353 1 402 2431 (fax 2764)
--------------------
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