st: Replacing missing values

["joe J." <otharain@hotmail.com>]

I have a panel (unbalanced) dataset of manufacturing plants for several 
years. Investment series is missing for a lot of plants. I tried the 
statacode -ipolate-, which uses linear interpolation and  extrapolation 
procedures. But these have resulted in many negative values.  Is there any 
way one could restrict -ipolate- from extrapolating negative values or are 
there superior methods to replace missing values?

Joe

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