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

Stata 11 Sample Session Section 2 – Restructuring Data Files – Table Lookup & Aggregation
conver.dta file or “using” data file, (using conver.dta as
a table lookup) to add the conver variable to our
working data file. We had renamed p1a to unit.
Key variables are required in any procedure to merge two
files when one of the files is being used as a keyed table.
Our key variables specify how to merge the lookup file
using product and unit (the grouping variables), because
we have a different conversion factor for each productunit
combination. If we had used only prod, Stata would
expect each product to have only a single conversion
factor, with the same value regardless of the unit of
measurement used. For example, it would expect the
same conversion factor for rice whether it was in a 100 kg
bag or a 20 liter can. This would be incorrect.
Check the resulting data file The new working file produced by the join contains the
needed conversion factor variable, conver. For every
product-unit combination, conver is equal to the number
of kilograms in that unit. It is always important to verify
if the join was successfully completed. Click on the Data
Browser button to look at some cases to verify that the
conversion factors match the products. We could also
use the list command to see if a 20-liter can filled with
maize grain has a conversion value of 18 kilograms (prod
= 47 unit = 8).
The Stata command is:
list prod unit conver if prod==47 & unit ==8
Note: Two equal signs (==) are required.
The two equal signs distinguish relational equality from
the =exp assignment phrase. For example, if you want to
create a variable where you will be assigning values to
that variable, you will use an expression (exp) and need
only 1 equal sign (example: gen newvar = oldvar*2.5).
In the above example, prod already has values and we
want to see only records where prod has the value of 47.
Therefore, it is a relational equality and we must use 2
equal signs (e.g. show me only records where prod ==47
and unit == 8).
We should also run a tabulate on the _merge variable as
well to look at how the merge was done:
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tab1 _merge