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

Stata 11 Sample Session Section 2 – Restructuring Data Files – Table Lookup & Aggregation
Compute total kilograms
produced
The generate command
From the output you should see there are the same
number of records in the file as there was before the
merge, i.e. 1,693. Note that there are 27 cases where
there was not a match for the prod-unit combination in
the look-up file. How would you specify the “list”
command to look at these 27 cases?
You would want to investigate further to see if the
records without a look-up value are crops that you want
to have included in the analysis you are doing, and, if
they are, correct the lookup file and/or the production file
and run your procedure again.
We can now calculate total kilograms by multiplying the
number of units (p1b) by the conversion factor.
1. Select Create or change variables from the
Data menu
2. Select Create new variable
The Generate - Create a new variable dialog box
opens.
3. Under the Main tab, change the Variable type to
“double”.
4. Type the name of the new variable in the
Variable name box: qprod_tt
5. For the Contents of new variable box, type in
p1b * conver
6. Click on the copy button, switch to the do-file
editor, paste the command, switch back to the
dialog box and click on Ok
7. Add a comment in the do-file to explain what
you have done.
The Stata command is:
generate double qprod_tt= p1b * conver
Note that there were 49 cases where no value was
generated. We had 27 cases with no conversion value.
Why do the rest of the cases (22 cases) not have values?
Now that the kilograms have been calculated we need to
look up the value of a kilogram in calories for each
product. This information is in the table lookup file
called calories.dta. This file has two variables —
product and number of calories per kilogram. The key
variable is product (prod). In order to add the calorieconversion
variable to the working data file we need to
do another merge with keyed table lookup (joinby). This
time the key variable only needs to be the product
variable. The data file has already been sorted by product
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