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RE: st: Using weights from the Current Population survey


From   Lee Sieswerda <Lee.Sieswerda@tbdhu.com>
To   "'statalist@hsphsun2.harvard.edu'" <statalist@hsphsun2.harvard.edu>
Subject   RE: st: Using weights from the Current Population survey
Date   Wed, 3 Jul 2002 14:23:56 -0400

A better option than using iweight or aweight with -tabulate- is to use
-svytab- and pweight. It has several advantages, one of which is that gives
you asymptotic 95% CIs that will not cross 0 or 1 (which is good because you
can't have less than 0 people enter a survey response, nor more than 100%). 

And to head off the next question, -svytab- won't allow you to specify only
one variable in the command statement. If you want to tabulate the responses
to a single question you have to create a constant (e.g., gen dum=1) and use
it as your second variable, as in:
svytab q1 dum, [options - lots of them]

Also, make sure you use the -subpop- option to specify subpopulations, not
-if- or -in-.

And finally, you absolutely have to use the weights provided if you want the
results to be representative of the population and not just the sample. I
don't know about the CPS specifically, but it is unlikely that you'll be
provided with the psu and strata information because in well-conducted
surveys the clusters sampled are usually quite small and it would be
possible for a determined analyst to identify individuals - especially in
lightly-populated areas. To get around this problem in Canada, at least, for
large government surveys we are provided with a data set of bootstrap
weights from which to calculate bootstrapped standard errors. If you do not
have access to the psu and strata information, then you might enquire if
such a beast is available for the Current Population Survey.

Best regards,

Lee S.

Lee Sieswerda, Epidemiologist
Thunder Bay District Health Unit
999 Balmoral Street
Thunder Bay, Ontario
Canada  P7B 6E7
Tel: +1 (807) 625-5957
Fax: +1 (807) 623-2369
Lee.Sieswerda@tbdhu.com
www.tbdhu.com



> -----Original Message-----
> From:	David Kantor [SMTP:dkantor@jhu.edu]
> Sent:	Wednesday, July 03, 2002 1:53 PM
> To:	statalist@hsphsun2.harvard.edu
> Subject:	Re: st: Using weights from the Current Population survey
> 
> At 12:18 PM 7/3/2002 -0500, Nammi Kandula wrote:
> >I am doing an analysis with the MARCH current population survey.
> >I am doing a person-level analysis.
> >
> >If I use the wgt varaible, stata asks me what kind of weight this is.
> >Is it an analytic weight, pweight, fweight?
> >
> >Should I use the weight in my regressions, or in my tab commands. Do i
> need
> >to transform the weight in any way?
> 
> My experience is that all weights in surveys from the U.S. Census Bureau 
> are pweights.
> 
> You should specify them as pweights in regressions. (If you use them as 
> aweight, the coefficients will be the same, but the variances and 
> confidence intervals will be wrong.  See the section in the user guide on 
> Estimation, Weighted Estimation; that's U 26.12 in my ancient V5 manual.)
> 
> Better -- use svyreg and specify the strata and psu, if these are 
> identified. Still, specify the weight as pweight.
> 
> For -tabulate-, pweight is not accepted.  Use aweight or iweight; the 
> proportions will not be affected by the choice, but iweight has the 
> advantage that the "Freq." will be the weighted sums of the observations
> -- 
> i.e., the estimated number of actual population in the given category.  Be
> 
> sure you have scaled the weight correctly at the outset, if there are any 
> implied decimals in the raw data.
> 
> I hope this helps.
> -- David K.
> 
> David Kantor
> Institute for Policy Studies
> Johns Hopkins University
> dkantor@jhu.edu
> 410-516-5404
> 
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