Stata The Stata listserver
[Date Prev][Date Next][Thread Prev][Thread Next][Date index][Thread index]

Re: st: more cases than controls

From   Ron�n Conroy <[email protected]>
To   "statalist" <[email protected]>
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 [email protected] 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 ([email protected])
Lecturer in Biostatistics
Royal College of Surgeons
Dublin 2, Ireland
+353 1 402 2431 (fax 2764)

Just say no to drug reps

This email and any files transmitted with it are confidential and
intended solely for the use of the individual or entity to whom
they are addressed.
If you have received this email in error please notify the
originator of the message. This footer also confirms that this
email message has been scanned for the presence of computer viruses.

Any views expressed in this message are those of the individual
sender, except where the sender specifies and with authority,
states them to be the views of The Royal College Of Surgeons in Ireland.


*   For searches and help try:

© Copyright 1996–2024 StataCorp LLC   |   Terms of use   |   Privacy   |   Contact us   |   What's new   |   Site index