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Re: st: more cases than controls


From   Michel Camus <mcamus@videotron.ca>
To   statalist@hsphsun2.harvard.edu
Subject   Re: st: more cases than controls
Date   Mon, 22 Mar 2004 22:37:43 -0500

Ricardo Ovaldia wrote:

(...) We recently submitted a manuscript for publication to
a major medical journal. It was a case-control study
with 329 cases and 126 controls. One of the reviewers
wrote that "to have such a larger number of cases was
statistically atypical" and asked if the "authors find
that the use of the same control for multiple patients
significantly limits results"?

I never heard of any biases or other problems cause by
having more cases than controls in a study. We had
sufficient power and the difference for our main
outcome was highly significant (less than 0.00001). Am
I missing something or is it that this reviewer does
not understand the case-control designed? By the way
this was not a matched study design.
Thank you,
Ricardo.

Dear Ricardo,
There is no problem per se with having less controls than cases, though it should raise some eyebrows.
The critique of using "the same control for multiple patients" suggests the reviewer's misunderstanding of an unmatched design. A smaller number of controls for a single group of cases is "atypical" still.
One usually chooses an equal or larger group of controls to increase power to be able to detect even a small odds ratio when exposure is relatively rare.
A smaller number of controls than cases suggests that the investigators had more cases than needed given an expected a priori a large relative risk (>5) and a high prevalence of exposure (>75%) among controls (cf. Schlesselmann, 1982, p.155). Could it not then be construed that the investigators knew enough beforehand not to do a study?...
With respect to the outcome measure, I do not understand how you can say from a case-control study that "the difference for our main outcome was highly significant (less than 0.00001)". Usually the measure of effect in a case-control study is an odds ratio, not a difference (in rates?).
Michel

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Michel Camus, Ph.D.

Épidémiologue, Div. Biostatistique et épidémiologie, DGSESC, Santé Canada

Epidemiologist, Biostatistics and Epidemiology Div., HECSB, Health Canada

Courriel / e-mail : mcamus@videotron.ca <mailto:mcamus@videotron.ca>

Téléphone / phone : (514) 850-0157

Télécopieur / fax : (514) 850-0836

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