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st: generate new variable from differencing where subtracted variable is fixed


From   Johannes Schoder <johannes.schoder@soi.uzh.ch>
To   statalist@hsphsun2.harvard.edu
Subject   st: generate new variable from differencing where subtracted variable is fixed
Date   Fri, 05 Mar 2010 17:00:06 +0100

Dear Statalist users:

I try to calculate the difference between two variables for a panel data set, where the subtracted variable (here x2) is always the first observation of each year:
For example:
Year   x1   x2         difference: x1-x2 if x2 is the first obs. of the year

1960   5     8           5-8
1960   4     10         4-8
1960   2     3           2-8
1961   5     7           5-7
1961   1     2           1-7
1961   3     5           3-7


I tried something like: bysort year: gen difference= x1-x2[_n==1] but Stata does only calculate the first observation for each year and otherwise generates missings.
Thanks a lot for any hint!
Regards,
Johannes

--


Johannes Schoder
Socioeconomic Institute
University of Zurich
Hottingerstr. 10
8032 Zurich

Phone: +41 44 634 0612
Fax:   +41 44 634 4987
E-mail: johannes.schoder@soi.uzh.ch

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