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AW: AW: st: PSMATCH with 2 conditions


From   "Mihai-Andrei Popescu-Greaca" <mischai@gmx.de>
To   <statalist@hsphsun2.harvard.edu>
Subject   AW: AW: st: PSMATCH with 2 conditions
Date   Mon, 20 Sep 2010 23:50:46 +0200

Dear Judson,

Just tried out the multiplication with 10 and 1000 on the nlswork dataset,
and again, the T-values are different (without bootstrapping). When I use
pscore2 (multiplication with 10) I get a T of 9.16, Difference is .102809747
and SE is .011229545. Using pscore3 (*1000) yields a T of 4.02, a Difference
of .092227641 and a SE of .02294341.

The only explanation I came up with is that (my) Stata "looses" decimals
with each multiplication or performs a "mysterious" round-up. I.e. if my
initial pscore is 0.28732124 (like row 3 in the dataset), then the pscore2
is 720.28729 and the pscore3 is 72000.287. IF my theory holds, then by
multiplying with a huge number, all decimals should disappear. I generated a
pscore4 (*1000000) and now I only see 72000000, but T is still 4.72,
Difference is .263281083 and SE .055774089, hence this doesn't make sense
either.

I'm using Stata 11.1..I really don't know what else can be wrong..:(((((

PS: I exported the data to excel, and only the "short" numbers get exported
(i.e. 72000.287, and NOT 72*1000+0.287321424)

Best,
Mihai

-----Ursprüngliche Nachricht-----
Von: owner-statalist@hsphsun2.harvard.edu
[mailto:owner-statalist@hsphsun2.harvard.edu] Im Auftrag von Caskey, Judson
Gesendet: Montag, 20. September 2010 17:48
An: statalist@hsphsun2.harvard.edu
Betreff: re: AW: st: PSMATCH with 2 conditions

Mihai,

I was unable to replicate the problem. Can you try replicating it on one of
the public datasets and send an example?

Here is what I tried:

webuse nlswork
logit union collgrad age tenure not_smsa c_city south nev_mar
predict pscore if e(sample), pr
gen pscore2=year*10+pscore 
gen pscore3=year*1000+pscore
psmatch2 union, pscore(pscore2) outcome(ln_wage) caliper(0.5) 
psmatch2 union, pscore(pscore3) outcome(ln_wage) caliper(0.5)


I get the same results in both of the psmatch2 calls. The issue may be with
the bootstrapping procedure rather than with the call to psmatch2, itself.

Regards,

Judson

----------------------------------

Hi Judson,

thanks for the great tip & sorry for the late reply but I've been pretty
busy lately.

One thing though:
My industry is only 2 digit, so if I only want to match by industry, I just
multiply industry by 10 and then add the pscore, as follows:
gen pscore2=3-digit-industry*10+pscore
And then psmatch2 as indicated by you in the link.
THE PROBLEM occurred when I used 1000 instead of 10, and then 100000 instead
of 1000; ALL 3 yielded different results (T & Z-scores when bootstrapping
SEs)

Do you have an explanation for the different results??

Regards,
Mihai

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