Re: st: sampsi and percentages

[Ricardo Ovaldia <ovaldia@yahoo.com>]

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


----- Original Message -----
From: Nick Cox <njcoxstata@gmail.com>
To: statalist@hsphsun2.harvard.edu
Cc: 
Sent: Tuesday, August 30, 2011 10:02 AM
Subject: Re: st: sampsi and percentages

Two points:

1. In terms of your example, length/weight is not always < 1. The
value of that ratio is crucially dependent on some choice of units of
measurement. Suppose I measure my (wife's) car's length in centimetres
and its weight (mass) in tonnes, for example.
You can call this pedantry but I react to incorrect statements!

2. More importantly, if something is bounded by (0,1) -- can we take
that pair of () literally as implying 0 < data < 1? -- then it will
behave like a proportion regardless of how the calculation was done.
For example, an average very near 0 can only be achieved if all values
are near 0 and so the variance will be very small, and similarly for
an average near 1. However, that may not help much.

Nick

On Tue, Aug 30, 2011 at 3:45 PM, Ricardo Ovaldia <ovaldia@yahoo.com> wrote:
>
> 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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