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Re: st: A little problem with clustered data

From   John Taffe <>
Subject   Re: st: A little problem with clustered data
Date   Mon, 03 May 2004 09:57:17 +1000

Ronan, it's unfortunate that depression scale is confounded with hospital. Were hospitals randomly assigned to scales? Are other known characteristics of hospitals possible confounders? Or factors related to how the scales were presented? It seems unfortunate also that a depression scale should be seen as 'threatening'. Perhaps a cognac to go with the pipes.

John Taffe

Ronán Conroy wrote:

I would like to show you the results of two logistic regressions and get
ideas on how to carry the analysis forward.

The situation is this: a survey was run in 38 hospitals. The survey used two
depression scales. Half the hospitals received the first scale, the other
half the second.

Not all patients completed a depression scale. The researcher suspected that
one of the scales was less likely to be returned (it was more threatening
than the other). So he ran a logistic regression, which confirmed his

. logistic depress_scale_ok which_scale

Logistic regression Number of obs = 1206
LR chi2(1) = 13.09
Prob > chi2 = 0.0003
Log likelihood = -758.99238 Pseudo R2 = 0.0086

depress_sc~k | Odds Ratio Std. Err. z P>|z| [95% Conf. Interval]
which_scale | 1.560516 .1927593 3.60 0.000 1.22497 1.987975

However, when he used -svyset- to set the PSU to hospital, to account for
patient clustering within hospitals, this is what happens (same point
estimate, but much wider confidence intervals)

. svylogit depress_scale_ok which_scale if which_scale <3, or

Survey logistic regression

pweight: <none> Number of obs = 1206
Strata: <one> Number of strata = 1
PSU: hospital_number Number of PSUs = 38
Population size = 1206
F( 1, 37) = 3.15
Prob > F = 0.0839

depress_sc~k | Odds Ratio Std. Err. t P>|t| [95% Conf. Interval]
which_scale | 1.560516 .390986 1.78 0.084 .9392774 2.592642

So it would seem that the variation between hospitals in the rate of return
is greater than the variation you would expect from a binomial process. This
accords with the researcher's experience. Some hospitals took a dislike to
the depression scales, and this was more likely to happen with the more
threatening one.
My question, finally, is what next? Clearly, one source of variability in
the return of completed depression scales is whether the hospital thinks
that it is a useful exercise or not. But are hospitals allocated the second
scale more likely to withhold their collaboration? How much of the poorer
return rate is the unwillingness of patients to fill in the scale, and how
much is the way in which the hospital handles the task of administering the
scale and making sure it is returned?

A good two-pipe problem, as my old metaphysics tutor used to say.

Ronan M Conroy (
Lecturer in Biostatistics
Royal College of Surgeons
Dublin 2, Ireland
+353 1 402 2431 (fax 2764)

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