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Re: st: sampsi and percentages


From   Ricardo Ovaldia <ovaldia@yahoo.com>
To   "statalist@hsphsun2.harvard.edu" <statalist@hsphsun2.harvard.edu>
Subject   Re: st: sampsi and percentages
Date   Tue, 30 Aug 2011 07:45:28 -0700 (PDT)

 
Thank you, but these are not proportions. They are intensity measures. You can think of them as ratios of two continous things. 
For example with the auto data, they could be the ratio of car's length  to weight (length / weight) which is always between 0 and 1.
Now less say that you want to compare these ratio between between foreign and domestic cars.
 
Ricardo

Ricardo Ovaldia, MS
Statistician 
Oklahoma City, OK


----- Original Message -----
From: "Ariel Linden, DrPH" <ariel.linden@gmail.com>
To: statalist@hsphsun2.harvard.edu
Cc: 
Sent: Tuesday, August 30, 2011 7:51 AM
Subject: re: st: sampsi and percentages

Ricardo,

I may be mistaken here, but it seems you have two proportions (if it's
bounded between 0,1 then you have a numerator and a denominator for each
group).

If that is truly the case, you can use sampsi for proportions:

. sampsi 0.25 0.4

Estimated sample size for two-sample comparison of proportions

Test Ho: p1 = p2, where p1 is the proportion in population 1
                    and p2 is the proportion in population 2
Assumptions:

        alpha =  0.0500  (two-sided)
        power =  0.9000
            p1 =  0.2500
            p2 =  0.4000
        n2/n1 =  1.00

Estimated required sample sizes:

            n1 =      216
            n2 =      216

I hope this helps

Ariel

Date: Mon, 29 Aug 2011 11:40:33 -0700 (PDT)
From: Ricardo Ovaldia <ovaldia@yahoo.com>
Subject: st: sampsi and percentages



I need to compute sample size and power for a study comparing two group on a
measurement bounded by (0,1), (a measure of intensity).
I was thinking about using -sampsi- to power on the difference of means.
However, this seems strange to me, is there another way to power such
comparison?

Thank you,
Ricardo

Ricardo Ovaldia, MS
Statistician 
Oklahoma City, OK



    

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