Re: st: Is a "merge, if" possible?

[Phil Schumm <pschumm@uchicago.edu>]

On Oct 20, 2009, at 4:47 PM, Polis, Chelsea B. wrote:
> So, from within the dataset with three variables, I want to write  
> something like:
> merge study_id using "C:\Round 1.dta", if lastneground==1
> merge study_id using "C:\Round 2.dta", if lastneground==2
> and so on through round 10.
>
> But, merge doesn't seem to be able to be combined with an if  
> statement.  Are there any elegant ways of doing this, or must I do  
> it piecemeal (creating what I need in 10 separate datasets and  
> appending)?

On Oct 20, 2009, at 4:58 PM, Nick Cox wrote:
> The help for -merge- would tell you if -if- were allowed, and it is  
> not.

<snip>

> -append- can readily combine several datasets, unless you are not in  
> Stata 11, in which case you should have told us that.


To add to what Nick said, this can be done in 2 lines of code.  The  
first appends the Round 1, Round 2, etc. files, and uses the - 
generate()- option to create a new variable called lastneground  
containing the values 1, 2, etc. according to the source file.  The  
second uses -merge- to perform a one-to-one merge with your subject- 
level file using the keys study_id and lastneground; here, you'll want  
to specify -keep(match)- (or perhaps -keep(match using)-) to retain  
one record per subject.


-- Phil

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