Introduction to Stata 8

10.6. Combining files [U] 25
Appending files [R] append
To combine the information from two files with the same variables, but different persons:
// c:\dokumenter\proj1\gen.filab.do
use c:\dokumenter\proj1\fila.dta , clear
append using c:\dokumenter\proj1\filb.dta
save c:\dokumenter\proj1\filab.dta
Merging files [R] merge
To combine the information from two files with different information about the same persons:
// c:\dokumenter\proj1\gen.filab.do
use c:\dokumenter\proj1\fila.dta , clear
merge lbnr using c:\dokumenter\proj1\filb.dta
save c:\dokumenter\proj1\filab.dta
Both files must be sorted beforehand by the matching key (lbnr in the example above), and
the matching key must have the same name in both data sets. Apart from the matching key the
variable names should be different. Below A and B symbolize the variable set in the input
files, and numbers represent the matching key. Missing information is shown by . (period):
fila filb filab _merge
1A
2A
4A1
4A2
1B
3B
4B
1AB
2A.
3.B
4A1B
4A2B
3
1
2
3
3
Stata creates the variable _merge which takes the value 1 if only data set 1 (fila)
contributes, 2 if only data set 2 (filb) contributes, and 3 if both sets contribute. Check for
mismatches by:
tab1 _merge
list lbnr _merge if _merge < 3
For lbnr 4 there were two observations in fila, but only one in filb. The result was two
observations with the information from filb assigned to both of them. This enables to
distribute information eg. about doctors to each of their patients – if that is what you desire.
But what if the duplicate lbnr 4 was an error? To check for duplicate id's before merging,
sort and compare with the previous observation:
sort lbnr
list lbnr if lbnr==lbnr[_n-1]
Another way to check for and list observations with duplicate id's is:
duplicates report lbnr
duplicates list lbnr
merge is a lot more flexible than described here; see [R] merge.
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