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re: Re: st: sampsi and percentages


From   "Ariel Linden, DrPH" <ariel.linden@gmail.com>
To   <statalist@hsphsun2.harvard.edu>
Subject   re: Re: st: sampsi and percentages
Date   Wed, 31 Aug 2011 09:28:01 -0400

I agree with Nick on the need to run simulations. The obvious problem you
have is that there are too many moving parts. If you took the mean of the
ratio, it would not be accounting for the fact that you could achieve the
same ratio by moving the numerator and denominator up or down in opposing
directions.

Perhaps you could hold one of the variables (numerator or denominator)
constant (at the mean or mode, etc.) and run the sample size calculations,
then hold the other variable constant and rerun. If you get similar sample
sizes, you know that you're not far off.

I'd be very cognizant of the standard deviations as well, since I am not
sure how large or small they'll be, given that you say the ratios are
bounded.

Ariel 
Date: Tue, 30 Aug 2011 20:08:11 +0100
From: Nick Cox <njcoxstata@gmail.com>
Subject: Re: st: sampsi and percentages

Sure. I was partly in jest, but as a scientist I never feel
constrained by the particular units in which data arrive, especially
if they are not even metric units, let alone natural. Your example
remains units-dependent in that numerator and denominator have quite
different units. Not important unless this is also true of your real
example.

The best way forward for you is likely to be not looking for a canned
approach but simulating datasets of different sizes under plausible
generating processes and seeing what is or is not detectable.

Nick

On Tue, Aug 30, 2011 at 7:33 PM, Ricardo Ovaldia <ovaldia@yahoo.com> wrote:
>
> Thank you Nick. I was specificaly talking about how lenght and weight are
recorded in the auto data.
>
>  gen r= length / weight
> . sum r
>     Variable |       Obs        Mean    Std. Dev.       Min        Max
> -------------+--------------------------------------------------------
>            r |        74    .0647308    .0102566   .0475524   .0872222
>
> The ratio is less that one in all observations!
> So the statement was not incorrect.
>
> Regarding your second point: Yes it behaves as a proportion but I cannot
use a sample size calculation for proportion to power this study because
there is not a true denominator.
> Which brings me back to my original issue of how to power this study.
>
> Ricardo.
>
> Ricardo Ovaldia, MS
> Statistician
> Oklahoma City, OK

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