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 <[email protected]> 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.
>
> �pid�miologue, Div. Biostatistique et �pid�miologie,
> DGSESC, Sant� Canada
>
> Epidemiologist, Biostatistics and Epidemiology Div.,
> HECSB, Health Canada
>
> Courriel / e-mail : [email protected]
> <mailto:[email protected]>
>
> T�l�phone / phone : (514) 850-0157
>
> T�l�copieur / fax : (514) 850-0836
>
> ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~
> ~ ~ ~ ~ ~ ~ ~ ~ ~ ~
> ~ ~ ~ ~ ~
> ==============================
>
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