st: Re: Dealing with strata with singleton PSU's

["R.E. De Hoyos" <redeho@hotmail.com>]

Trish,

If you are using Stata 8 or under, what I would do is the following:

1. Identify the singletons for each of your age/gender cell using Jeff 
Pitblado's -singleton-
(code provided)

***** BEGIN: singleton.ado
program singleton, sort
version 8
syntax [varlist(numeric default=none)] [if] [in], ///
STRata(varname) gen(name) [ PSU(varname) ]
confirm new var `gen'
marksample touse
if "`psu'" == "" {
tempvar psu
gen `psu' = _n
}
tempvar u
sort `touse' `strata' `psu'
quietly by `touse' `strata' `psu': gen `u' = _n == 1
quietly by `touse' `strata': replace `u' = sum(`u')
quietly by `touse' `strata': replace `u' = cond(`u'[_N] == 1, 1, 0)

quietly replace `u' = . if !`touse'
rename `u' `gen'
end
***** END: singleton.ado

2. Create a general-singleton identifier:

gen gsingleton = singleton_cell1 | singleton_cell2 | ...

3. Collapse the strata to remove the singletons identified by "gsingleton". 
This will allow you to have the same stratification for each of your 
statistics. The pweights must remain the same, since the expanssion factors 
are associated with the PSU's and these wont change.

I hope this helps,

Rafa
________________________
R.E. De Hoyos
Faculty of Economics
University of Cambridge
CB3 9DE, UK
www.econ.cam.ac.uk/phd/red29/

----- Original Message ----- 
From: "Trish Gorely" <P.J.Gorely@lboro.ac.uk>
To: <statalist@hsphsun2.harvard.edu>
Sent: Thursday, May 19, 2005 2:30 PM
Subject: st: Dealing with strata with singleton PSU's



> I have a stratified data set that I want to calculate means and proportions
> for using svymean and svyprop.  Unfortunately I have some
> strata with single PSU's and svymean and svyprop don't like this.  The
> manual and help service recommend 2 ways of dealing with the singleton
> PSU's:
> 1. collapse across strata to effectively remove them (the advice being to
> collapse in the way that makes most sense for your data)
> 2. drop the singleton PSU's
>
> The preferred option for me is to collapse across strata and I can do this
> easily enough. However I'm still not clear on the following:
>
> 1.  do you need to recalculate probability weights?
> 2.  Do you need to use the same collapsed strata for everyone?  For 
> example,
> when I do svymean for Grade 9 boys I have 3 singelton PSU but when I do 
> the
> same analysis for Grade 10 boys there are 4 singleton PSU's, and at grade 
> 11
> 7! The problem is much less in girls (grade 9 there is one, grade 10 1 and
> grade11 3).  Should I collapse to remove the singleton's at year 11 boys
> (which would, by chance have the net effect of removing all the singletons
> at all year/gender groups) calling the new strata NEWSTRA, and then use
> NEWSTRA to define the data for all analyses, or should  I be doing the
> relevant collapse for each age/gender group?
>
> Thanks for any help anyone can offer
> Trish
>
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