-ipolate- interpolates linearly within gaps. That
is, it is assumed that the y variable varies linearly with
the x variable within any gaps. This is best seen
geometrically, as the last (x,y) pair before any gap
and the first (x,y) pair after any gap are just joined
by a straight line and intermediate results read
off directly.
In addition, a consequence of the assumption that
y is piecewise linear in x is
that repeated y's at any x are just averaged.
Plotting your results will make it easier to
see what is going on.
The help file is somewhat elliptical here.
Nick
n.j.cox@durham.ac.uk
Jann Ben
> Stata's -ipolate- command produces results I don't
> understand. Here is an example:
>
> . set seed 2346
> . sysuse auto
> . drop if rep78<. & uniform()<.8
> . ipolate rep78 mpg, g(rep78i)
> . sort mpg
> . list rep78 mpg rep78i, clean
>
> rep78 mpg rep78i
> 1. . 14 .
> 2. 4 15 3.5
> 3. 3 15 3.5
> 4. 3 16 3
> 5. 2 17 3.5
> 6. 5 17 3.5
> 7. 3 18 3
> 8. 3 19 3
> 9. 3 19 3
> 10. . 19 3
> 11. 3 21 3
> 12. . 22 3.5
> 13. 4 23 4
> 14. 4 25 4
> 15. 4 25 4
> 16. . 26 4.1
> 17. . 26 4.1
> 18. 5 35 5
>
> -help ipolate- states that rep78i should equal rep78
> if rep78 is not missing ("ipolate creates newvar = yvar,
> where yvar is not missing"). This is certainly not the case
> in the above example. For some reason, "3.5" is stored
> for cases 2, 3, 5, and 6. Can someone explain this to me?
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