STATA 11 for Windows SAMPLE SESSION - Food Security Group ...

Stata 11 Sample Session Section 2 – Restructuring Data Files – Table Lookup & Aggregation
Rename any key variables in both
files to the same name
each case in the production file (c-q4.dta), we need to
look up the product and unit in the conver.dta file. We
will merge the information from this file into the file in
memory (the production file). The variable with the
conversion factor will then be available to calculate the
total kgs produced. In Stata we want to use the “joinby”
command for this merge. It can be found through the
menus with the following choice:
Data
Combine datasets
Form all pairwise combinations within groups.
The input files for a merge must be sorted by the key
variable(s) (key variables are those variables you are
using to match by between the two files). Since there is a
unique conversion factor for each product-unit
combination, both our product variable and our unit
variable are the key variables. The CONVER.DTA file is
already sorted by prod and unit. We must sort the
current working file that is in memory the same way,
while taking account of the fact that the unit variable is
named p1a and not unit. To sort the cases:
1. From the Data menu select
Sort
Ascending data
The Sort - Sort data dialog box will open.
2. In the Variables: box select prod and p1a
3. Click on the “copy” icon and then click on Ok.
4. Switch to the do-file editor and paste the command.
The Stata command is:
sort prod p1a
Let’s look at the two variables using the tab1 command.
We can type in the Command window
tab1 prod p1a
There are 1,693 cases. We have many products. For the
tabulation of p1a we see 2 values that have no labels (0
and 1) and note that there are only 1670 cases that contain
a value for p1a. There are possible data problems. We
would expect to see a value in p1a for every crop that was
harvested. How would you determine if there are missing
data in the p1a variable? If it were possible, corrections
should be made before proceeding further.
We cannot merge the two files unless the variables that
57