Re: st: Correctly setting FPC in svyset

[Steven Samuels <sjsamuels@gmail.com>]

The finite population correction is not appropriate here. The theory  
for it applies only for the sampling fraction set by the design: e.g..  
if you'd had a population of 1,000 and selected 500 at random. In  
contrast, you tried to do a census and select 100%. The schools that  
responded are not a random sample of the total.

Your problem is how to deal with the large bias due to non-response.  
For some partial remedies, consult the sections on non-response in a  
good sampling text. I suggest especially Lohr, Sampling Design and  
Analysis, Duxbury, 1999 or 2010 and Groves et al. 'Survey  
Methodology", Wiley, 2010. Both contain further references.

In my experience, trying to do a census is almost always a mistake.  
You would have had better information (smaller mean square error) from  
a designed sample of 50 to 200 schools with methods intended to  
minimize non-response. We've needed such methods even in business  
surveys where participation was required by law. If it were my study,  
I would contact a sample of the non-responders, and I suggest that you  
do that if possible.


Steve

Steven J. Samuels
sjsamuels@gmail.com
18 Cantine's Island
Saugerties NY 12477
USA
Voice: 845-246-0774
Fax:    206-202-4783

On Feb 9, 2011, at 6:44 AM, R G wrote:

Hello,

I'm analyzing a census I've done of schools, no stratification or  
clustering.
Response rate is around 50%. I need to set the FPC (finite population
correction) in svyset and I'm not sure the appropriate number to use.  
I know
some argue that there's no need to set the FPC, but in my case the  
issues I'm
surveying on there is a strong case for considering a finite  
population, and not
a superpopulation.

According to Stata help:

fpc(varname) requests a finite population correction for the variance
estimates.  If varname has values less than or
       equal to 1, it is interpreted as a stratum sampling rate f_h =  
n_h/N_h,
where n_h = number of units sampled from
       stratum h and N_h = total number of units in the population  
belonging to
stratum h.  If varname has values greater
       than or equal to n_h, it is interpreted as containing N_h.  It  
is an
error for varname to have values between 1
       and n_h or to have a mixture of sampling rates and stratus  
sizes.

The UCLA page also states that:
Stata will calculate the actual fpc for us; we just need to  specify the
population total.
(Source:
http://www.ats.ucla.edu/stat/stata/seminars/svy_stata_8/Stata_svy_seminar_sampling.htm)


If there are a total of 1000 schools, with 500 responses, am I correct  
in
interpreting that I should set the fpc=1000?

Just need to confirm this, as I'm relatively new at Stata's svy  
commands.

Thanks

Robin



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