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st: combining multiple years of data, take two...


From   <nora@uchicago.edu>
To   statalist@hsphsun2.harvard.edu
Subject   st: combining multiple years of data, take two...
Date   Fri, 25 May 2007 12:00:18 -0500 (CDT)

Hello. 

I wrote earlier this year about how to svyset a data set
involving multiple years of data from the CDC's BRFSS survey.
I got a response (posted below) for what to do. This seemed to
work fine, but now when I try to do a regression using
svyregress, I get an error message saying "data not set up for
svy, use survey set." any ideas what could be wrong? 

Thanks
Eleanor Friedman 


--------------------------------------------------------------
Eleanor Friedman <nora@uchicago.edu> asks for the Stata
equivalent of using the PSULEV option in SUDAAN's NEST statement:


Hello, I am a first time poster, so please indulge me!I am
using STATA 9 to try and examine trends across multiple
years of data from the CDC's BRFSS survey. I have had no
problem when using a single year of data using the SVY
commands. the CDC offers the following SUDAAN code to combine
years: "NEST SURVYEAR _STSTR_PSU PSULEV=3;" My problem is that
I have no idea how to translate this into STATA. As far as I
have been able to determine, NEST does not exist in STATA, and
I am unable to find an analog. I contacted the CDC asking how
to combine several years of BRFSS data in STATA  and they said
I should contact people who use STATA more frequently.
Suggestions?


In the SUDANN line

	NEST SURVYEAR _STSTR_PSU PSULEV=3;

the 'PSULEV=3' option identifies the position of the PSU
(primary sampling unit) variable in the NEST statement.  This
means that all other variables to the left of this position
identify the strata in the first stage.  Eleanor's
example only specifies 2 variables, so this means that the
'SURVYEAR' and '_STSTR_PSU' variables identify the first stage
strata.

Although Stata's -svyset- command allows only one variable in
the -strata()- option for each sampling stage, Eleanor can use
-egen- with the -group()-function to generate a single strata
variable for use with -svyset- in this case.

Assuming Eleanor only has information for the first stage and
the weight variable (in her Stata dataset) is named
SAMPWEIGHT, she can do the following:

	. egen STRATA1 = group(SURVYEAR _STSTR_PSU)
	. svyset _n [pw=SAMPWEIGHT], strata(STRATA1)

I would also suggest that Eleanor save this modified dataset
under a new name,such as

	. save mybrfss

Then Eleanor can use this new 'mybrfss.dta' Stata dataset in
the future without having to worry about the survey design
variables since the -svyset- information is saved with the
data in 'mybrfss.dta'

--Jeff
jpitblado@stata.com
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