Re: [R] sample size estimation for count (poisson?) data?

From: Shawn Morrison <shawn.morrison_at_ualberta.ca>
Date: Thu, 13 Nov 2008 14:43:39 -0700

Thanks. I did the search before I posted and found those threads. However, it does not seem to do what I want. All I want to do is estimate the sample size for a point estimate, not do a GLM. I just want the mean within a margin of error, and to a given CI.

I've tried writing some code to do a simulation (below). Will this do the job?

#Generate data from Poission distribution, with lambda = 5 data = rpois(200, lambda = 5)
mean(data); var(data)

#Parameter Estimates
moe = 0.03 # margin of error = +/- 3%
sample.size = 168 # number of hunters to sample

#Draw sample size from population, calc mean. Run 10,000 iterations d = numeric(10000)
for (i in 1:10000) {
samp = (sample(data, sample.size, replace = FALSE)) d[i] = mean(samp)
}

#What are the bounds on the values that correspond to the margin of error?
lower=mean(data)-moe
upper=mean(data)+moe

#values from 'd' based on 90% confidence intervals q25=quantile(d, 0.05)
q95=quantile(d,0.95)

#top row = bounds on the mean from the margin of error, second row = bounds based on simulated data and sample size, third row = 1 = true, 0 = false in terms of the sample size being adequate to meet requirements of the margin of error.
output=rbind(cbind(lower,upper), cbind(q25,q95), cbind(q25>lower, q95<upper))
row.names(output) = c("known", "estimated","True/False") output

On 12-Nov-08, at 4:41 PM, David Winsemius wrote:

> The first hit for search on "sample size" and "poisson" on Baron's
> search engine web interface appears on target:
>
> http://search.r-project.org/cgi-bin/namazu.cgi?query=%22sample+size%22+poisson&max=100&result=normal&sort=score&idxname=functions&idxname=Rhelp02a
>
> Getting the same result from your console window requires a couple
> of extra back-slashes:
>
> > RSiteSearch(""sample size" poisson")
> Error: syntax error
> > RSiteSearch("\"sample size\" poisson")
> A search query has been submitted to http://search.r-project.org
> The results page should open in your browser shortly.
>
> --
> David Winsemius
> Heritage Labs
>
>
> On Nov 12, 2008, at 2:46 PM, Shawn Morrison wrote:
>
>> Is there a function in R that will allow me to estimate the sample
>> size required from count data (poisson data?), given the known
>> variance and desired margin of error and confidence interval?
>>
>> My specific data set will be based on a survey of hikers that will be
>> asked about the number of animals of species 'x' they observed during
>> a given period. I need to know the number of hikers to interview. ie,
>> I would like to calculate the mean number of species 'x' +/- margin
>> of
>> error with 95% confidence.
>>
>> This is a simple exercise for normally distributed continuous data,
>> but I'm running into roadblocks for count data.
>>
>> Sincerely,
>> Shawn Morrison
>> [[alternative HTML version deleted]]
>>
>> ______________________________________________
>> R-help_at_r-project.org mailing list
>> https://stat.ethz.ch/mailman/listinfo/r-help
>> PLEASE do read the posting guide http://www.R-project.org/posting-guide.html
>> and provide commented, minimal, self-contained, reproducible code.
>

        [[alternative HTML version deleted]]



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