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Re: st: Sample size calculations


From   Joerg Luedicke <joerg.luedicke@gmail.com>
To   statalist@hsphsun2.harvard.edu
Subject   Re: st: Sample size calculations
Date   Thu, 14 Mar 2013 11:13:02 -0400

As Nick said, it does not seem to be quite clear what exactly you
would like to do. Are the data that you have some kind of pilot data
and you are planning a larger study? In any case, your input
information is not sufficient:

a) The effect sizes you mention are meaningless without knowing
anything about the variance of your outcome measure. That is why
people often use some flavor of standardized effect sizes. Simply put,
if your outcome variance (or better yet: the variance of the Gaussian
error in your assumed linear model) is very large, then an effect of 7
might be quite small, whereas if the variance in the outcome is small,
7 might be quite large.

b) You don't say what exactly these effects are. Are these differences
between certain ethnic groups? Differences to some reference values?

c) What do you mean with "parameter p"?

In any case, stuff like this is probably best done with a bunch of
simulations. Perhaps have a look at Al Feiveson's article:

http://www.stata-journal.com/article.html?article=st0010

and go from there.

Joerg


On Thu, Mar 14, 2013 at 10:01 AM, Amal Khanolkar <Amal.Khanolkar@ki.se> wrote:
> Hi,
>
> I plan to run a set of multivariate linear regressions models which will help analyze ethnic differences in my outcome of interest.
>
> regress Y i.ethnicity age i.education height bmi bloodpressure
>
> I have 14 different ethnic groups.
>
> As some of my ethnic groups are fairly small I would like to calculate the sample size needed for each of the fourteen ethnic groups. I have a rough idea of the expected effect or parameter p in each ethnic group.
>
> I've only calculated sample sizes for a two group comparison using the sampsi command in Stata.
>
> Does anyone know how I could go about testing expected sample sizes for several categories of the exposure variable?
>
> I would set other parameters needed to calculate sample size as follows: alpha=0.05, power (0.90), expected effect for most ethnic groups varies between -2.85 to -7.65
>
> Thanks & regards
>
> Amal
>
>
>
>
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