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st: Issues with merging data


From   "Anibal Santos" <ssrnmail@gmail.com>
To   <statalist@hsphsun2.harvard.edu>
Subject   st: Issues with merging data
Date   Tue, 16 May 2006 00:12:15 +0100

Hi everyone,

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?

Best,

AS

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