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Re: st: merging datasets


From   Steve Samuels <sjsamuels@gmail.com>
To   statalist@hsphsun2.harvard.edu
Subject   Re: st: merging datasets
Date   Mon, 20 Feb 2012 09:05:43 -0500

Kyleigh Schraeder:

As a start, I assume that by "small sample", you mean a small survey sample, with its own survey structure. If not, see below.  The trick is to combine the two data sets in such a way so that you can -svyset- the combination and apply Stata's survey commands. This is the standard approach for combining two independent samples.

Suppose the -svyset- statement for large survey data set is:

************************
svyset psu [pw = myweight], strata(stratumvar)   
************************

In what follows, I use the prefix "small_" for variables from the small data set. If the small sample was not a survey sample, then for "small_psu" use "_n" and for "small_weight" use "1".  It is crucial to assign a stratum number for the small sample that is not present in the large sample.

**************************************
use small_data, clear
gen psu = small_psu  
gen int stratumvar = small_stratum+10000  // a stratum number not in the large sample
gen myweight = small_weight
gen samptype = 1
append using large_data   
replace samptype = 2 if sample==.
label define samptype 1 "small" 2 "national"
label values samptype samptype

svyset psu [pw = myweight], stratum(stratumvar)
save combo, replace
**************************************

Then do any survey command for comparing the two samples, e.g.

************************
svy: reg myoutcome i.samptype
*************************

Hypothesis tests are often not appropriate for comparing descriptive statistics of two finite populations, because no specific groups of people can expected to have _identical_ means or other statistics.  For references, see: http://www.stata.com/statalist/archive/2011-09/msg01121.html. Confidence intervals provide a satisfying alternative.


Steve
sjsamuels@gmail.com


On Feb 19, 2012, at 6:03 PM, Kyleigh Schraeder wrote:

I would like to compare means and percentages from a small sample
dataset to a large population national dataset.  The population
dataset has population weights.  I have stset the population dataset
but I need to merge the two datasets in order to run analyses.  I am
wondering how I can merge the two datasets but still apply the
population weights to the large dataset beforehand (and not apply the
population weights to the small sample).

Thanks for your help,
KS
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