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st: RE: Issues with merging data


From   "Nick Cox" <n.j.cox@durham.ac.uk>
To   <statalist@hsphsun2.harvard.edu>
Subject   st: RE: Issues with merging data
Date   Tue, 16 May 2006 00:24:39 +0100

It seems to me that you got from
Stata what you asked for. What 
you want requires a further -drop-
using -_merge-. 

Nick 
n.j.cox@durham.ac.uk 

Anibal Santos
 
> I'm trying to merge two datasets: one with accounting data for several
> companies and the other with monthly stock returns and market 
> value. I'm
> using the following command:
> 
> . merge dscd Year Month using "C:\DATA\mktdata\ret_mv.dta", 
> unique sort
> With the accounting data loaded in memory.
> After running this command I edited the merged file and found 
> that stata
> used all the observations from ret_mv.dta, even though the 
> accounting data
> file only has information for one Year and one Month per 
> observation. Ex.:
> 
> Accouting data
> Id	Year	Month	TotalAssets
> 1	1990	6	1000
> 1	1991	6	1100	
> 1	1992	6	1200
> 2	1995	6	200
> 2	1996	6	400
> 2	1997	6	500
> ...
> 
> Ret_mv.dta
> 
> Id	Year	Month	RET	MV
> 1	1990	1	.1	100
> 1	1990	2	.2	120
> 1	1990	3	.1	132	
> 1	1990	4	0	132
> 1	1990	5	.1	155
> 1	1990	6	0	155
> 1	1990	7	.2	185
> 1	1990	8	0	185
> ...
> 
> The after running the merger command I get the following:
> 
> Id	Year	Month	TotalAssets	RET	MV
> 1	1990	1	.		.1	100
> 1	1990	2	.		.2	120
> 1	1990	3	.		.1	132	
> 1	1990	4	.		0	132
> 1	1990	5	.		.1	155
> 1	1990	6	1000		0	155
> 1	1990	7	.		.2	185
> 1	1990	8	.		0	185
> ...
> 
> But the results should have been:
> Datamerged
> Id	Year	Month	TotalAssets	RET	MV
> 1	1990	6	1000		0	155
> 1	1991	6	1100		x	y
> 1	1992	6	1200		xx	yy
> ...
> 
> Can anyone explain why this happens and if there is any way 
> of solving this issue?

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