Re: st: RE: psmatch2 with the if condition

[Roberto Mura <robertomura2@robertomura.com>]

Nick,
Thank you for your kind reply. The loop is not the main issue. Even 
if I do not specify any loop but simply code:
use "C:\AAA.dta", clear
catenate condition1 =  country sic2
gen quotedcondition1=1 if quotedcompany==1
replace quotedcondition1=0 if quotedcompany==0 & diversification_orig==1
gen quotednumerator=.
gen quoteddenomenator=.
gen privatenumerator=.
gen privatedenominator=.
egen g=group(condition1)
bysort condition1: gen Ncondition1=_N

psmatch2 quotedcondition1 if g==1, pscore(size) noreplacement

I still find that psmatch2 uses companies which do not belong to 
group 1 to look for matches. As a matter of fact, it is assigning _id 
values outside the range of group 1 (there are just 15 firms in it 
but psmatch2 reports values like 877). Also, I am not blaming 
psmatch2 for anything and I never said I discovered a bug. I am just 
trying to understand what I am doing wrong. Also, please note that my 
code is very similar to the one reported in the help of psmatch2 
which I report below for simplicity:
Matching within strata
    The following code illustrates how to match within exact cells 
and then calculate the average effect for the whole population.

        g att = .
        egen g = group(groupvars)
        levels g, local(gr)
        qui foreach j of local gr {
                psmatch2 treatvar varlist if g==`j', out(outvar)
                replace att = r(att) if  g==`j'
        }
        sum att
Kind regards
R




At 11:22 22/03/2010, you wrote:
> You refer to various user-written programs here. Remember that you are
> asked to explain where user-written programs mentioned in a post come
> from.
>
> -catenate- is from SSC. It has long since morphed into, and been
> superseded by, the official -egen- function -concat()-. However, its use
> here is not problematic.
>
> More importantly, -psmatch2- is a very often used command from SSC. I
> haven't used it, but my guess is very much that you are misunderstanding
> something rather than that you have discovered a bug.
>
> Note that Stata is not doing anything independently of the commands you
> issue, contrary to some of your wording here.
>
> I haven't tried to understand your detailed logic, but what seems much
> more likely than a problem with -psmatch2- is that you are
> misunderstanding what your -while- loops are doing.
>
> You have two loops
>
> local j=1
> while `j' <= g {
>         ...
>         local i=1
>         while `i' < Ncondition1 {
>                 ...
>         }
> }
>
> Both involve looping until a value stored in a variable is reached, -g-
> in the outer loop and -Ncondition1- in the inner loop. With this kind of
> programming what Stata does is look at the value of the named variable
> _in the first observation_. This is documented at
>
> FAQ     . . . . . . . . . . . . . . . . . . . . .  if command vs. if
> qualifier
>         . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  J.
> Wernow
>         6/00    I have an if command in my program that only seems
>                 to evaluate the first observation, what's going on?
>                 http://www.stata.com/support/faqs/lang/ifqualifier.html
>
> So, it's as if you had programmed
>
> local j=1
> while `j' <= g[1] {
>         ...
>         local i=1
>         while `i' < Ncondition1[1] {
>                 ...
>         }
> }
>
> It's evident that is definitely not what you want and leading to your
> loop terminating before the work you want is done and that you are
> unjustly putting the blame on -psmatch2-. It may be that your loop just
> goes round once and does not get beyond -g[1]- which is 1.
>
> Nick
> n.j.cox@durham.ac.uk
>
> Dr. Roberto Mura, Ph.D. [edited]
>
> I have a cross section of companies, some listed some private.
> I am trying to find a matching private (undiversified) firm for each
> listed one, on the basis of company size. Also, since my dataset is
> cross country, I am trying to control for country and SIC code as
> well. To this end, I have created a condition, called -condition1-, to
> divide the sample into a set of clusters:
>
> catenate condition1 =  country sic2
>
> Now, once a match is found, I need Stata to report some accounting
> data for the matched private firm on the same row as the  listed firm.
> I have tried to create a "self-fulfilling" nested loop so that I
> don't need to specify beforehand the total number of clusters
> (-condition1-) and also because the total number of firms changes with
> different clusters (-Ncondition1- below).  The problem that I am facing
> is that -psmatch2- does not seem to understand the -if- condition.
> I have uploaded the dataset (for version 11 and for version 9 of
> Stata) and the do file in my website under
> www.robertomura.com/psmatch2/ if you want to take a look.
> Here is what I do:
>
> use "D:\AAA.dta", clear
> catenate condition1 =  country sic2
> gen quotedcondition1=1 if quotedcompany==1
> replace quotedcondition1=0 if quotedcompany==0 & diversification_orig==1
> gen quotednumerator=.
> gen quoteddenomenator=.
> gen privatenumerator=.
> gen privatedenominator=.
> egen g=group(condition1)
> bysort condition1: gen Ncondition1=_N
>
> local j=1
> while `j' <=g {
> psmatch2 quotedcondition1 if g==`j', pscore(size) noreplacement
> replace quotednumerator=fnetincome if _nn==1
> replace quoteddenomenator=shareholdersfundsnew if _nn==1
> replace privatenumerator=fnetincome if _n1==_id
> replace privatedenominator=shareholdersfundsnew if _n1==_id
>
> local i=1
> while `i' <Ncondition1 {
> replace privatenumerator=fnetincome[_n+`i'] if _n1==_id[_n+`i']
> replace privatedenominator=shareholdersfundsnew[_n+`i'] if
> _n1==_id[_n+`i']
> replace privatenumerator=fnetincome[_n-`i'] if _n1==_id[_n-`i']
> replace privatedenominator=shareholdersfundsnew[_n-`i'] if
> _n1==_id[_n-`i']
> local i = `i' + 1
> }
> local j = `j' + 1
> }
>
>
> The problem seems to lie in
>
> psmatch2 quotedcondition1 if g==`j', pscore(size) noreplacement
>
> What seems to be happening is that the matching is done only for the
> first 3 firms of the first cluster and then Stata is mis-assigning
> the _id since the numbers do not follow the clustering but rather,
> Stata is applying the -psmatch2- considering the sample as a whole. If,
> for instance, I change the if condition into:
>
> psmatch2 quotedcondition1 if g==1, pscore(size) noreplacement
>
> then Stata is not matching inside the cluster (condition1) but again
> it is treating the sample as a whole. In other words the "if"
> condition does not appear to be used at all by Stata.
>
> I have tried both version of psmatch2. The one available through SSC
> (version 3.1.5 2may2009) and the more recent one available from
> http://leuven.economists.nl/stata (4.0.1)
> I have Stata/MP 11.0 for Windows (64-bit x86-64)  running on XP Pro
> 64. I also tried with the older version I had, Stata/SE 9.2 for
> Windows 64-bit x86-64and things don't change.
>
>
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