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st: RE: Sample size


From   VISINTAINER PAUL <VISINT@NYMC.EDU>
To   "'statalist@hsphsun2.harvard.edu'" <statalist@hsphsun2.harvard.edu>
Subject   st: RE: Sample size
Date   Mon, 28 Apr 2003 12:09:57 -0400

Don,

The problem you are having with sample size is that you haven't given enough
information.  It isn't clear whether you want to simply estimate the
prevalence/incidence of a condition in the population; whether you want to
"test" whether the occurrence in the population is really .001, or whether
you want to test the difference between groups, assuming the occurrence in
general is .001.  The last two options require you to specify an alternative
hypothesis, which you haven't given.  

Using your sampsi input, you are specifying a comparison between a
prevalence of 1 per 1000 vs. none (or a really very, very rare prevalence).
In this case you're specifying that the null value is .001 and your
alternative is that it is much more rare than that.  If you reverse your
figures (e.g., sampsi 0 .001, p(.8)) you're specifying that the null value
is near 0 and your alternative hypothesis is that it is much more prevalent.


(I was actually surprised that sampsi performed the calculation with 0 as an
entry.  I suppose it actually uses a very small value for 0.) 

For the first option, you rather just estimate the prevalence of this
condition, (which you think is pretty rare at .001), you might want to focus
on the precision of the estimate by specifying the width of the confidence
interval.  I don't think we can get a sample size estimate based on the
width of a confidence interval using sampsi.  

So, what do you want to do?

Paul


-----Original Message-----
From: Don Spady [mailto:dspady@ualberta.ca] 
Sent: Monday, April 28, 2003 11:19 AM
To: Statalist
Subject: st: Sample size

Dear all
  I sent this before but got no response.  I have revised it.
I want to estimate the sample size needed to detect an disease that
occurs in 1 out of 1000 people (as an example).   The alternate
state is absence of disease which would occur in 999 of 1000 people on
average.   The problem is that I get numbers but I don't know if they
are the
right ones.  Can I use sampsi grp1 being those with disease and Grp2
being
those without disease.  Or do I use sampsi   0.001, onesample as in:

sampsi 0.001 0, p(0.8) onesample

I need help and thank in advance those that provide it.

Donald Spady
Dep't of Pediatrics, University of Alberta
(780) 407-1244

Nature has no reset button.

Donald Spady
Dep't of Pediatrics, University of Alberta
(780) 407-1244

Nature has no reset button.


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