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# RE: st: ratio of 2 different surveys estimates? svyset , suest , fpc , nlcom

 From "Roman Kasal" <[email protected]> To <[email protected]> Subject RE: st: ratio of 2 different surveys estimates? svyset , suest , fpc , nlcom Date Thu, 20 Jan 2011 10:03:54 +0100

```ok, I understand that, I already did this once, but is this
statistically correct adjusting=combine FPCs of two different surveys?
What if the FPCs are really very different (this is not mentioned case)?

-----Original Message-----
From: [email protected]
[mailto:[email protected]] On Behalf Of Steven
Samuels
Sent: Wednesday, January 19, 2011 9:08 PM
To: [email protected]
Subject: Re: st: ratio of 2 different surveys estimates? svyset , suest
, fpc , nlcom

Another way of putting this: When you combine independent surveys, you
must create a new stratum variable in which the individual original
strata in each survey appear. You would do this in your data, for
example by:
****************************
egen stratum =group(year stk)
*****************************
If there were 10 strata in 2006 and 10 in 2007, the new stratum
variable should have 20 levels. Then a single -svyset- statement will
allow joint analysis of the two surveys, as in my example.

Steve
[email protected]

On Jan 18, 2011, at 8:52 PM, Steven Samuels wrote:

I don't understand all your manipulations, Roman, but I think that the
error message provides the key: the same stratum numbers appear in
both years, but with different fpcs,, depending on the year.  Renumber
the strata so that this doesn't happen.

*************************
webuse income, clear
gen  year = 2006 + (famid>68)

gen stratum = 1 if  famid<=34                    //2006
replace stratum =2 if  inrange(famid,35,68)      //2006
replace stratum =3 if  inrange(famid, 69, 100)   //2007
replace stratum =4 if inrange(famid, 101,135)    //2007

gen fpc = .05*(stratum==1) +.10*(stratum==2) +.12*(stratum==3) +.
15*(stratum==4)

svyset famid, strata(stratum) fpc(fpc)
svy: reg inc if year==2006
estimates store y2006
svy: reg inc if year==2007
estimates store y2007
suest y2006 y2007
matrix list e(V)   //results from different years are independent
nlcom  _b[y2006:_cons]/_b[y2007:_cons]
***********************************************

Steve

Steven J. Samuels
[email protected]
18 Cantine's Island
Saugerties NY 12477
USA
Voice: 845-246-0774
Fax:    206-202-4783

On Jan 18, 2011, at 3:03 AM, Roman Kasal wrote:

I have 2 estemates in 2 surveys with two different FPCs (finite
population corrections). I would like to calculate ratio "nlcom
[r6]_cons/[r7]_cons" but:

"
for year 2006
svyset ico [pw=vahaHMM], strata(stk) fpc(vb1) singleunit(centered)
svy: reg X if year==2006
estimates save r2006

for year 2007
svyset ico [pw=vahaHMM], strata(stk) fpc(vb1) singleunit(centered)
svy: reg Y if year==2007
estimates save r2007

estu use r2006
estimates esample:
est sto r6

estu use r2007
estimates esample:
est sto r7

suest r6 r7

and error:
fpc for all observations within a stratum must be the same
_robust2 failed to compute cross model variance estimate
likely an internal error in suest

"

1) is any way how to easily handle this correctly in STATA? Because two
different FPCs are correct for 2 years. Within 1 year (2006 or 2007) the
FPCs are well set because "svy: reg X if year==2006" is working.
2) do I have to calculate the ratio manually? Do you know any tools or
3) when I know the fpc does not differ much, can I use average FPC for
the both surveys for the ratio (not for averages)?

thank you

Roman Kasal

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```