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# Re: st: stpower cox with a binary covariate

 From Anders Gaarsdal Holst <[email protected]> To [email protected] Subject Re: st: stpower cox with a binary covariate Date Fri, 18 Feb 2011 08:11:03 +0100

```Of cause. Guess I should have been able to figure that out by my self.
Thank you very much.

Sincere regards,
Anders Gaarsdal Holst
MD, Research Fellow
Department of Cardiology, B2142
University Hospital Rigshospitalet
Blegdamsvej 9
2100 Copenhagen

Phone: +45 35450639
Email: [email protected]

On Thu, Feb 17, 2011 at 9:16 PM, Steven Samuels <[email protected]> wrote:
>
> Anders, you calculated the SD incorrectly:
>
> It should be: (.1*(1-.1))^.5 = .09^.5 = 0.30
>
> Steve
> [email protected]
>
> On Feb 17, 2011, at 2:15 PM, Anders Gaarsdal Holst wrote:
>
> Dear Statalist,
>
> I'm trying to do a sample size calculation for a study we are
> planning. In the study we are going to examine the cardiac rhythm
> disorder atrial fibrillation (binary covariate) as a predictor for
> stroke (endpoint) and analyze the data in a Cox ph model. We are
> probably going to analyze it with atrial fibrillation as a time
> dependent covariate but for power calculation purposes I regard the
> covariate as non time dependent.
>
> The probability of having a stroke in the follow up period is assumed
> to be 4%, the prevalence of atrial fibrillation is assumed to be 10%
> and the desired detectable difference is set to a HR of 3.
>
> When I try to do as described in the Stata 11 manual, I calculate the
> SD of my covariate as:
>
> . di 0.1*(1 -0.1)^0.5
> .09486833
>
> This gives me following:
> . stpower cox, failprob(0.04) hratio(3.00) sd(0.095) hr
>
> Estimated sample size for Cox PH regression
> Wald test, hazard metric
> Ho: [b1, b2, ..., bp] = [0, b2, ..., bp]
>
> Input parameters:
>
>        alpha =    0.0500  (two sided)
>       hratio =    3.0000
>           sd =    0.0950
>        power =    0.8000
>    Pr(event) =    0.0400
>
> Estimated number of events and sample size:
>
>            E =       721
>            N =     18015
>
> This sample size seems very high, and I think I am doing something
> wrong, but what?
>
> Sincere regards,
> Anders Gaarsdal Holst
> MD, Research Fellow
> Department of Cardiology, B2142
> University Hospital Rigshospitalet
> Blegdamsvej 9
> 2100 Copenhagen
>
> Phone: +45 35450639
> Email: [email protected]
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```