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RE: st: Interpolation using cipolate


From   "Nick Cox" <n.j.cox@durham.ac.uk>
To   <statalist@hsphsun2.harvard.edu>
Subject   RE: st: Interpolation using cipolate
Date   Wed, 14 Apr 2004 21:06:35 +0100

Good. But others might appreciate
the explanation of what -cipolate- 
is. Among other advice, the Statalist
FAQ suggests 

Say what command(s) you are using. If they are not part of 
official Stata, say where they come from: the STB/SJ, SSC, 
or other archives. 

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

joe J.
 
> Thanks Nick. I got the awakening when Richard gave the reply.
> 
> >-cipolate- is a user-written command on SSC for cubic
> >interpolation. As Richard surmises, the logic is that
> >you must interpolate gaps in a response with respect
> >to a second variable (e.g. time), just as with official
> >Stata's -ipolate-, which is for linear interpolation.
> >
> >Richard Williams replied to Joe J.
> >
> > > >A naive question. I just insalled cipolate to fill missing
> > > values for one
> > > >variable. The documentation tells me that interpolation is
> > > done using two
> > > >points before and two points after the missing value. That
> > > said, why is it
> > > >that the procedure requires a second variable? In other 
> words, for
> > > >replacing missing values in variable y, why we need to
> > > include variable x
> > > >in the command?
> > > >
> > > >. cipolate y x, gen(yprime).
> >
> > > I don't know about -cipolate-, but the docs for -ipolate-
> > > (reference manual
> > > G-M, pp. 182-183) say that Y is a function of X.  The value for
> > > interpolated Y is computed using values from the surrounding
> > > Xs and Ys that
> > > are not missing.
> > >
> > > If you are going to interpolate, your data have to be sorted
> > > in some way,
> > > right?  So having an X accomplishes that.  The distance
> > > between Xs will be
> > > related to the interpolated differences between Ys.

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