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From |
austin nichols <austinnichols@gmail.com> |

To |
statalist@hsphsun2.harvard.edu |

Subject |
Re: st: RE: Problems with -ipolate- |

Date |
Tue, 6 Sep 2005 09:38:52 -0400 |

While I can see Nick's point about interpolation in general, I agree with Ben Jann that -ipolate- should not "replace" nonmissing values of the y variable when it interpolates using multiple y values per value of x, since according to its own help file it does not. The missing value of rep78 for mpg==19 should be filled in, but not the four nonmissing values of rep78 for mpg==15 or 17. I would add the following to line 75 of ipolate.ado (currently a blank line): qui replace `z'=`usery' if `usery'<. and alter the first two lines to read *! version 1.0 6sep2005 based on ipolate, version 1.3.3 21sep2004 program define nipolate, byable(onecall) sort then save the revised program as nipolate.ado, which produces rep78 mpg rep78i . 14 . 4 15 4 3 15 3 3 16 3 5 17 5 2 17 2 3 18 3 3 19 3 . 19 3 3 19 3 3 21 3 . 22 3.5 4 23 4 4 25 4 4 25 4 . 26 4.1 . 26 4.1 5 35 5 in Ben's example. Alternatively, you could add an option to line 5, making it e.g. */ [ BY(varlist) Epolate noavg] and make line 75: if "`avg'"!="" { qui replace `z'=`usery' if `usery'<. } so nipolate would behave as does ipolate unless you specify noavg. On 9/5/05, Nick Cox <n.j.cox@durham.ac.uk> wrote: > -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 > > > Ben Jann wrote: > > > > 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? * * For searches and help try: * http://www.stata.com/support/faqs/res/findit.html * http://www.stata.com/support/statalist/faq * http://www.ats.ucla.edu/stat/stata/

**References**:**st: RE: Problems with -ipolate-***From:*"Nick Cox" <n.j.cox@durham.ac.uk>

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