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Re: st: more cases than controls


From   Philip Ryan <philip.ryan@adelaide.edu.au>
To   statalist@hsphsun2.harvard.edu
Subject   Re: st: more cases than controls
Date   Wed, 24 Mar 2004 09:55:47 +1030

Ricardo

Leaving aside the question of relative numbers of cases and controls, I wonder if the reviewers remarked on your choice of analysis. That is to say, in a case control study the outcome is the case control status, not the antecedent exposure (in your study the biomarker level). A t-test reverses this sense of the study design, as the exposure is now the outcome and the case control status is (I was taught) forced unnaturally to be the "predictor". In modelling terms, keep the outcome defined by the study design on the left hand side. I would choose a logistic model, either keeping the biomarker level continuous if you believe there is a linear dose response with the log odds or perhaps with dummies of ordered categories of the biomarker if you wish to explore the functional nature of the relationship.

Phil




At 05:32 AM 23/03/2004 -0800, you wrote:

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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Department of Public Health
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