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st: RE: Replacing missing values


From   "Nick Cox" <n.j.cox@durham.ac.uk>
To   <statalist@hsphsun2.harvard.edu>
Subject   st: RE: Replacing missing values
Date   Thu, 22 Apr 2004 11:22:02 +0100

You have first to consider the possibility 
that your data do _not_ lend themselves
to linear interpolation. If, for example, 
investment varies irregularly, as seems 
plausible given that is a response to both 
macroeconomic and microeconomic factors, then 
gaps in a very peaky time series will be difficult to 
interpolate accurately. 

That said, so long as all your data are 
positive, interpolating on the logarithms and 
exponentiating the result is a sure 
mathematical way of avoiding negative predictions. 

Real economists will be able to add more. My 
economics education stopped with A-level (British 
high school leaving qualification) Economics 
in 1969. 

Nick 
n.j.cox@durham.ac.uk 

joe J.
 
> 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?

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