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# Re: st: Sampling for proportion: characteristic of interest

 From Stas Kolenikov To statalist@hsphsun2.harvard.edu Subject Re: st: Sampling for proportion: characteristic of interest Date Fri, 24 Aug 2012 14:33:26 -0500

```OK, let's start again. What is given to you (target sample size,
target margin of error, target effect size for a given power, etc.),
and what is it that you want to compute (sample size to achieve a
given accuracy; effect size that you can detect with a given power;
etc.)?

--
-- Stas Kolenikov  ::  http://stas.kolenikov.name
-- Senior Survey Statistician, Abt SRBI  ::  work email kolenikovs at
srbi dot com
-- Opinions stated in this email are mine only, and do not reflect the
position of my employer

On Fri, Aug 24, 2012 at 2:11 PM, Ilian, Henry (ACS)
<Henry.Ilian@dfa.state.ny.us> wrote:
> Stas, thank you.
>
> The question was: when you need to specify a proportion to calculate a sample size, the proportion is of a characteristic of interest, but when you have several characteristics of interest, all of which are important, how do you specify the proportion.
>
> Your answer would suggest that if there are yes/no questions on the instrument, then I need to specify 50% and therefore use a larger sample size. When you say you specify 10%-20% for Likert/multinomial scales, what percentage do you use for the calculation?
>
> Henry
>
> -----Original Message-----
> From: owner-statalist@hsphsun2.harvard.edu [mailto:owner-statalist@hsphsun2.harvard.edu] On Behalf Of Stas Kolenikov
> Sent: Friday, August 24, 2012 2:58 PM
> To: statalist@hsphsun2.harvard.edu
> Subject: Re: st: Sampling for proportion: characteristic of interest
>
> What exactly is your question here? You have not really stated what do
> you want to determine. You don't seem to have much of a design/sample
> size issue here, if you are set on sampling a known number of cases.
>
> When I write study precision and power sections, I usually specify
> several levels of incidence -- e.g., 10%, 20% and 50%. The 50% would
> be relevant for the Yes/No answers on your instrument; the 10%-20%
> would be relevant for your Likert/multinomial scales.
>
> --
> -- Stas Kolenikov  ::  http://stas.kolenikov.name
> -- Senior Survey Statistician, Abt SRBI  ::  work email kolenikovs at
> srbi dot com
> -- Opinions stated in this email are mine only, and do not reflect the
> position of my employer
>
>
> On Fri, Aug 24, 2012 at 11:35 AM, Ilian, Henry (ACS)
> <Henry.Ilian@dfa.state.ny.us> wrote:
>> I need to take a quality-control sample for a case-reading study. A random sample of about 600 cases will be drawn and read, and I need a sample of those to be read by a second reader. The case-reading instrument has approximately 100 questions--the final number hasn't been determined yet. The response choices mostly form ordinal scales, although a few will be nominal, hence I'll be sampling for proportion.
>>
>> Everything I can find on sampling for proportion only considers the proportions for a single characteristic of interest. I have 100+ characteristics with different expected proportions. How do I approach this problem?
>>
>> Many thanks,
>>
>> Henry Ilian
>>
>>
>>
>>
>>
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