# RE: st: An allowance for clustering should only increase SE's?

 From "CDSC - Nichols, Tom" To "'statalist@hsphsun2.harvard.edu'" Subject RE: st: An allowance for clustering should only increase SE's? Date Mon, 18 Apr 2005 13:56:26 +0100

```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, 'ça ne l'empèche
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 (rconroy@rcsi.ie)
Senior Lecturer in Biostatistics
Royal College of Surgeons
Dublin 2, Ireland
+353 1 402 2431 (fax 2764)
--------------------
Just say no to drug reps
http://www.nofreelunch.org/

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