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Re: st: more cases than controls


From   Ricardo Ovaldia <ovaldia@yahoo.com>
To   statalist@hsphsun2.harvard.edu
Subject   Re: st: more cases than controls
Date   Tue, 23 Mar 2004 05:32:25 -0800 (PST)

Thank you Michel,

I would like to clarify two points:

1. We had more cases than controls because of
budgetary constrains. It was easier and less expensive
to enroll cases than controls.

2. The main outcome of interest was a serum biomarker
measured on a continuous scale and log transformed for
the analysis. A t-test was used to compare cases and
controls and therefore no OR computed.

Best,
Ricardo.



--- Michel Camus <mcamus@videotron.ca> wrote:
> 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
> 
> ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~
> ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ 
> ~ ~ ~ ~ ~
> 
> 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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