# st: Re: RE: Re: RE: Sample size

 From "Don Spady" <[email protected]> To <[email protected]> Subject st: Re: RE: Re: RE: Sample size Date Mon, 28 Apr 2003 11:52:49 -0600

```Paul
Thank you for your help.  I will give it a try.
Don
----- Original Message -----
From: "VISINTAINER PAUL" <[email protected]>
To: <[email protected]>
Sent: Monday, April 28, 2003 11:35
Subject: st: RE: Re: RE: Sample size

> The formula for estimating the sample size based on the width of a
> confidence interval for a proportion is:
>
> n = (z^2 * p * q)/(d)^2, where z is the alpha level and d is the
one-sided
> difference between p and the upper (or lower) limit.  For example, if
you
> expect a proportion of .35, and you want to be 95% sure that p is no
larger
> than .40 (given that p is .35).  So with z=1.96, p=.35, q=.65, and
d=(.40 -
>
> Interestingly, you can get this using the -cii- command.
>
> .cii 350 .35, the difference though is that these confidence intervals
are
> exact.
>
> In your case, I wouldn't use the normal approximation to the binomial
> because your proportion is quite rare.  You could use -cii- and try
> different sample sizes, while maintaining the same proportion, e.g.,
>
> .cii 1000 1,  will give a wide confidence interval
>
> .cii 10000 10, will get you a narrower one.
>
> (Note these are exact limits, not normal approximations)
>
> What sampsi does is model both alpha error and beta error.  As long as
you
> don't specify a value for an alternative hypothesis (i.e., all you are
> interested in is interval estimation) you don't need to model beta
error.
>
>
>
> Paul
>
>
>
>
> -----Original Message-----
> From: Don Spady [mailto:[email protected]]
> Sent: Monday, April 28, 2003 1:07 PM
> To: [email protected]
> Subject: st: Re: RE: Sample size
>
> Paul
>    Thanks for your reply.  Indeed I want to estimate prevalence, with
> the interval being from 0.0001 to 0.0003 or there abouts.  I was told
> that the prevalence of the disease was between 1:200 and 1:2000,
> possibly closer to the 1:200.  By shooting at 0.0005, I would get the
> worst case scenario.  The confidence interval is hard to guess (say
the
> real value is 1:200 and I test for 1:2000, how do I estimate a
> confidence interval)  If the presence or absence follows a poisson
> distribution, then the variance is 1:2000 and the SD is 0.0224, I
think.
> Does this make much sense.
>
> Don
>
>
> ----- Original Message -----
> From: "VISINTAINER PAUL" <[email protected]>
> To: <[email protected]>
> Sent: Monday, April 28, 2003 10:09
> Subject: st: RE: Sample size
>
>
> > 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:[email protected]]
> > 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.
> >
> > Dep't of Pediatrics, University of Alberta
> > (780) 407-1244
> >
> > Nature has no reset button.
> >
> > Dep't of Pediatrics, University of Alberta
> > (780) 407-1244
> >
> > Nature has no reset button.
> >
> >
> > *
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> >
>
>
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```