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# Re: st: How to svyset when strata are used in some groups and not others

 From "Louise Linsell" To Subject Re: st: How to svyset when strata are used in some groups and not others Date Mon, 05 Jul 2010 10:06:39 +0100

```Thank you for suggestions. We have already tried defining 9 strata; 6 for the common type of hospital, for which we used stratified random sampling with 6 strata,  and 1 stratum each for the other 3 types of hospital, for which we took all units.

However, in the model we had to specify a finite population correction (FPC=sqrt(1-n/N)) as we sampled 28 out of 87 units for the most common type of hospital.

Because we sampled ALL the units from the other 3 types of hospital we had to set the FPC to zero since n=N (which is specified as 1 in Stata as it requires you to specify n/N). This means that there are no variance estimates when we summarise any outcomes in the 3 less common types of hospital, because it thinks we have sampled the whole population within these hospitals (when in fact we took a consecutive number of patients over a period of 3 months).

LL

>>> Stas Kolenikov <skolenik@gmail.com> 02/07/2010 20:36 >>>
If Louise sampled other 3 types lumping them together, then Steve's
recommendation is appropriate. If sampling was performed within each
of those remaining types, then the strata variable will have 6 (strata
in the most common type of hospitals) + 3 (other types of hospitals) =
9 levels.

On Fri, Jul 2, 2010 at 11:18 AM, Steve Samuels <sjsamuels@gmail.com> wrote:
> Louise-- create a stratum variable with 7 values: 1-6 for the
> hospitals of the first type, and 7 for the other three types, and use
> that in the strata() option of -svyset-
>
> Steve
>
> On Fri, Jul 2, 2010 at 12:00 PM, Louise Linsell
> <Louise.Linsell@npeu.ox.ac.uk> wrote:
>> I have a dataset with 4 different types of hospital, and would like to compare binary outcomes between them using logistic regression.  However for the first type  (the most common), hospitals were divided into 6 strata (based on size and SES) and a random sample was taken from each strata.  For the other 3 types of hospital we sampled all hospitals. My question is, how to use the svyset command when a different sampling strategy was used in one group?
>>
>> LL
>>
>>
>>
>>
>> *
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>>
>
>
>
> --
> Steven Samuels
> sjsamuels@gmail.com
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--
Stas Kolenikov, also found at http://stas.kolenikov.name
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```