Re: st: more cases than controls

[Michel Camus <mcamus@videotron.ca>]

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.

�pid�miologue, Div. Biostatistique et �pid�miologie, DGSESC, Sant� Canada

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

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

T�l�phone / phone     :    (514) 850-0157

T�l�copieur / fax        :    (514) 850-0836

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