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Re: st: sampsi


From   "Eurich, Dean" <deurich@ualberta.ca>
To   "statalist@hsphsun2.harvard.edu" <statalist@hsphsun2.harvard.edu>
Subject   Re: st: sampsi
Date   Fri, 19 Aug 2011 08:07:33 -0600

Thanks, The study is really two separate studies. The first is what sample
size do we require to observe a 2% change in magnetic imaging brain scans
for controls and ALS patients over an 8 month period. Second, do changes
in MRI scan brain scans predict log term mortality.

As we will have a very high number of events in this group, I thought I
would start with the change within 8 months to see what kind of a starting
point of sample we would require. Will tackle the Cox sample next. Unless
you have a fact way of doing it all in one - if so I am all ears!

Thanks

D.



On 11-08-19 1:54 AM, "Phil Schumm" <pschumm@uchicago.edu> wrote:

>On Aug 18, 2011, at 3:18 PM, Eurich, Dean wrote:
>> Ok trying to do a sampsi calculation with 2 groups with 1 baseline and
>>1 follow-up measure. It is basically a before after analysis with a
>>control group. We are interested in the change score from baseline to
>>the 8 week follow-up in predicting death for example
>> 
>> We have pilot data in 16 people: group 1 are healthy people; Group 2
>>are people with Luo Gerhig's disease (values are MRI scans)
>> Baseline MEAN Values are as follows:
>> Group 1 =3D0.61 (SD =3D 0.03)
>> Group 2 =3D 0.55 (SD =3D 0.03)
>> 
>> After 8 weeks we found the following MEANS:
>> Group 1 =3D 0.61 (SD 0.03)
>> Group 2 =3D 0.539 (SD 0.03)
>> 
>> So we have a mean change of 0.011 with a SD of change =3D 0.019
>> 
>> For my sample calculation I am totally confused on what to use for my
>>m1 and m2 and SD.
>
>
>For testing the null hypothesis that the mean change from baseline to
>follow-up is the same in both groups, you want
>
>    sampsi `=0.61-0.61' `=0.55-0.539', sd1(0.03) pre(1) post(1) r01(0.8)
>
>which gives 63 subjects per group for 90 percent power at the two-sided
>0.05 level.  This, of course, is equivalent to
>
>    sampsi `=0.61-0.61' `=0.55-0.539', sd1(0.019)
>
>(i.e., a simple t-test on the change scores).
>
>Note that while it's great that you have the benefit of preliminary data,
>your sample size in this case is very sensitive to your estimated
>correlation between the baseline and follow-up measurements.  For
>example, dropping it from 0.8 to 0.7 increases your sample size from 63
>per group to 94 per group.  A correlation of 0.8 is pretty high (though
>not impossible), and so I'd probably want to use a somewhat lower
>correlation for my power calculation, since there is undoubtably some
>error around the estimate.
>
>I'm a bit concerned about your statement that "we are interested in the
>change score from baseline to the 8 week follow-up in predicting death,"
>because this sounds like a very different hypothesis, which would require
>a different sample size calculation.
>
>
>-- Phil
>
>
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