st: Difference between variables per year

["Maurits Meijers Baumeister" <m.mb@gmx.com>]

Dear all,   I am using a data set that shows party positions per election year for multiple countries.  I'm looking to create a variable that shows the difference of the party position of a particular election year with the party position of the previous election year.     Please note that elections in the data set mostly do not happen yearly, although sometimes they might.   Moreover, the election years will be the same for all parties in the same country but will differ per country.     How do I do this?     
Thanks a lot,     Maurits Meijers  PhD student at Hertie School of Governance 
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