Hello, I have data in the following style: Group Date NumberObservations Numb. Obs. Last Year M1XM 01/2008 2 . M1XM 01/2009 221 2 M1XM 02/2009 452 2 M1XM 03/2009 132 2 M1XM 05/2009 45 2 M1XM 01/2010 2 221 M3GL 01/2008 4 . M3GL 01/2009 15 4 M3GL 02/2009 45 4 M3GL 03/2009 13 4 M3GL 05/2009 12 4 M3GL 08/2009 1265 4 M3GL 10/2009 985 4 M3GL 12/2009 451 4 M3GL 01/2010 352 15 M3GL 02/2010 41 45 M3GL 03/2010 7 13 M3GL 04/2010 45 . M3GL 05/2010 62 12 Additional Informations: I need the data "Numb. Obs. Last Year". The data of the year 2008 does not contain a month, nevertheless are the data formated as date (mm/yyyy). For 2008 there are always 01/2008. I need the lagged observations from the month in the previous year. For 2009 I want to use for every month 01/2008. The Dataset is very large. I tried it with lag function LAG<n>(argument), but my n differs. I'm able to calculate for each observation the n with a macro, but then I fail. It would be great if anybody can provide me a solution or an approach of solving the problem. Thank you very much in advance! Emanuel
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