Stat Computing > Stata > Notes

Stata Class Notes
Splitting & Combining Files

1.0 Stata commands in this unit

. drop cases
keep cases
append
drop variables
keep variables
sort
merge

2.0 Demonstration and Explanation

2.1 Create two new datasets by dropping cases

. use hsb2, clear
drop if female==0
save hsbf

Use save hsbf, replace if the file already exists.

. use hsb2, clear
drop if female==1
save hsbm
tabulate female

We started with hsb2 which had 200 cases and created two new datasets hsbm and hsbf using the drop command. hsbm has only males (n = 91) while hsbf has only females (n = 109).

2.2 Create two new datasets by keeping cases

The exact same thing can be accomplished using the keep command instead of the drop command.

. use hsb2, clear
keep if female==0
save hsbm, replace
tabulate female

. use hsb2, clear
keep if female==1
save hsbf, replace
tabulate female

Note that drop if female==0 is equivalent to keep if female==1.

2.3 Create a single dataset by appending

. use hsbm, clear
append using hsbf

. tabulate female

The append command concatenates two datasets, that is, sticks them together vertically, one after the other. Normally, the append command would be followed by a save command, however in this case, we already have the hsb2 dataset which is the same as hsbm and hsbf appended together.

2.4 Create two new datasets by dropping variables

. use hsb2, clear
drop female-prog
sort id
save hsbv1
describe

. use hsb2, clear
drop read-socst
sort id
save hsbv2
describe

We started with hsb2 which had 200 cases and 11 variables. Using the drop command we created hsbv1 which has six variables and hsbv2 which also has six variables. The variable id appears in both datasets.

2.5 Create two new datasets by keeping variables

As before, the exact same thing can be accomplished using the keep command instead of the drop command.

. use hsb2, clear
keep id read-socst
sort id
save hsbv1
describe

. use hsb2, clear
keep id female-prog
sort id
save hsbv2
describe

This time drop female-prog is equivalent to keep read-socst.

2.6 Create a single dataset by merging

. use hsbv1, clear
merge id using hsbv2
describe

The merge command sticks two datasets together horizontally, one next to the other. Both datasets must be sorted by the merge variable, in this case, id, before being merged. Normally, the merge command would be followed by a save command, however in this case, we already have the hsb2 dataset which is the same as hsbv1 and hsbv2 merged by id.

3.0 Try the commands on your own

. use hsb2, clear
drop if female~=0
save hsbf, replace
use hsb2, clear
drop if female~=1
save hsbm, replace
append using hsbf

4.0 Can you answer this questions?

1. What would happen if you did the following?:
   use hsb2, clear
   append using hsb2

5.0 For More Information

• Statistics with Stata 5 Chapter 2 (18-22, 41-45)
• Stata Learning Modules
  Subsetting Data
  Combining Data