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Re: st: more cases than controls


From   Ronán Conroy <rconroy@rcsi.ie>
To   "statalist hsphsun2.harvard.edu" <statalist@hsphsun2.harvard.edu>
Subject   Re: st: more cases than controls
Date   Wed, 24 Mar 2004 11:05:59 +0000

on 24/03/2004 04:01, Ricardo Ovaldia at ovaldia@yahoo.com wrote:

> This was a population-based Case–Control Study
> comparing a specific enzyme in the serum of infected
> patients (cases) to that in healthy non-infected
> controls. We compared these levels using a t-test
> after log-tranforming the data. Is this
> incorrect?There are many similar studies in the
> literature. Am I to understand that they are all
> incorrectly analyzed?

There is a difference of model involved.

When you do a t-test, your are asking whether the presence of a condition
affects the mean value of a variable of interest. Do patients with
Parkinson's disease have poorer performance on complex problem solving
tasks? (Yes, they do.)

When you do a logistic regression, you are asking whether a variable changes
the odds of having a disease or condition. Does being a women change the
odds of having depression (yes it does). Note that you cannot turn this
example around. It makes no sense to ask if depression affects your chances
of being a woman. 

Some case control studies can meaningfully be analysed either way. People
who have recently had a heart attack are more depressed than people who have
not. But there is evidence that depression is not just a consequence of
serious physical illness, but actually a risk factor for heart disease. And,
of course, a case-control study cannot differentiate depression as a cause
of heart disease from depression as a sequel.

So you don't know the right statistical procedure until you know the
scientific model you are testing.


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

--------------------
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http://www.nofreelunch.org/

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