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Re: st: sampsi


From   Phil Schumm <pschumm@uchicago.edu>
To   statalist@hsphsun2.harvard.edu
Subject   Re: st: sampsi
Date   Fri, 19 Aug 2011 13:57:26 -0500

On Aug 19, 2011, at 9:07 AM, Eurich, Dean wrote:
> 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.


To clarify, the calculation you originally asked about involved detecting a difference in the mean change (in your imaging outcome) between ALS and controls.  IOW, it appears that you want to demonstrate that ALS patients on average decline faster than controls (who I presume are matched on age, gender, etc.).  This is consistent with the baseline measurements you showed (where the average for the controls was higher than for those with ALS), but I presume you want to show/estimate the difference longitudinally rather than just cross-sectionally.  You don't need to explain the rationale for doing this -- I just wanted to make sure that we were on the same page (your sentence above is a bit ambiguous WRT this).


-- Phil


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