I am trying to figure out how to get Stata to calculate the difference between values of a variable based on two observations but only when the subjects are the same. (Sort of a first differencing but with multiple subjects in the data set.)
Using the data set below, I have a fifth variable ('Diff') that is the difference between the values of 'Measure' for the same subject over the previous year. I want to have Stata calculate the difference between 109 & 105 (=4), 119 & 109 (=10) for Subject 1; and the difference between 105 & 90, 97 & 105 for Subject 2. Furthermore, since there is no value for Subject 1 prior to 2001 and prior to 2007 for Subject 2, there is no difference in the values of Measure over a previous year for those two subjects.
Obs Subject Year Measure Diff (Calc. result)
1 1 2001 105 n/a
2 1 2002 109 ? 4
3 1 2003 119 ? 10
4 2 2007 90 n/a
5 2 2008 105 ? 15
6 2 2009 97 ? -8
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