If you do not wish to interpolate the value before 2007 Dec, you can set them missing and then interpolation would begin from 2007 Dec. You said you found a good solution, would you please share it with me? thanks. Another question, as you described in https://communities.sas.com/thread/39579?start=0&tstart=0 If a lot of continuous missing values occurred in one time series, using proc expand will product the extreme value which is unreasonable. In my experience, using mean to fill will not make sense if the series have several high values. I try to discard the period having too long missing or fill the missing use the value in the same point (e.g., use 2011 Aug to fill 2012 Aug ).but until now, I do not have a good solution for it.
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