You have over a 100,000 subjects and you're worried about power? I doubt you're under-powered. Depending on what you're trying to do, you might want to split your sample for validation purposes, but that's a whole other topic.

Ryan

On Thu, Oct 13, 2011 at 3:29 PM, William Shakespeare <shakespeare_1040@hotmail.com> wrote: > This will be retrospective in nature using data already collected. The > questions I'm addressing all require logistic regression. I need to make > sure I have enough cases to adequately power my study but coming up with > the inputs for the sample size calculation is difficult. If this were > prospective I'd pilot to obtain the inputs. Due to the retrospective > nature of the present study the only way I could "pilot" it is to use the > existing data. I don't know if that's appropriate. > > On Thu, 13 Oct 2011 10:27:15 -0700, Fareeza Khurshed <fkhurshed@GMAIL.COM> > wrote: > >>Is it going to be a prospective study or a retrospective where you use > info >>from the db? >> >>On Thu, Oct 13, 2011 at 7:19 AM, William Shakespeare < >>shakespeare_1040@hotmail.com> wrote: >> >>> I have a large database (>100k subjects) which I'm going to use to > answer >>> several research questions. Normally, with a prospective study I'd run > a >>> pilot to obtain the necessary data for sample size calculation. > However, >>> that's not possible here. Would it be acceptable to draw a sample from > my >>> database and derive pilot data from it as long as I exclude all those > data >>> from subsequent analyses? >>> >