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From |
"Nick Cox" <n.j.cox@durham.ac.uk> |

To |
<statalist@hsphsun2.harvard.edu> |

Subject |
RE: st: RE: Imputed Missing Values with Uvis |

Date |
Fri, 25 Feb 2005 00:16:19 -0000 |

Thanks for this. It's a subtle point, but I'd say that -uvis- doesn't impute and it shows this by putting a missing value, rather than that it does impute and imputes with a missing value. By the rules of Stata it has to put something in the value of each variable. However, the implication is the same. Nick n.j.cox@durham.ac.uk Quang Nguyen > Dear Nick and Richard, > > Thanks so much for your advices. I highly appreciate it. Yes I do > understand that we need to impute "y" ONLY for observations with > missing values. Uvis, however, imputes "y" for all observations. As a > result, the imputed "y" might has missing value if any of "x" is > missing on a given observation. Thanks Richard for offering an > execellent way of dealing with this interesting issue. > > I truly enjoy this forum where any of my question is answered just > moments after it posted. You are GREAT! > > On Thu, 24 Feb 2005 23:36:07 -0000, Nick Cox > <n.j.cox@durham.ac.uk> wrote: > > No doubt this is what is often > > wanted. But the help for -uvis- > > is explicit: > > > > Note that uvis will not impute observations for which > > a value of a variable in xvarlist is > > missing. Only complete cases within xvarlist are used. > > > > Nick > > n.j.cox@durham.ac.uk > > > > Richard Williams > > > > > At 10:03 PM 2/24/2005 +0000, you wrote: > > > >You seem surprised at this. > > > > > > > >If you don't know -x1-, you cannot > > > >predict -y- from -x1 x2 x3-; and > > > >so on. > > > > > > > >If you do know -y- you don't need > > > >to impute it. > > > > > > If I understand Quang correctly, then the latter is exactly > > > the point. > > > -uvis- should have just plugged in the observed value for y > > > rather than > > > even try to impute it; but instead, it plugged in a missing > > > value since one > > > or more of the Xs for that case was missing. > > > > > > I tried -uvis- on a data set where y was not missing but some > > > values of x > > > were. The generated y had missing data whereas the original > > > y did not. > > > > > > I'm not that familiar with the programs but if nothing else > > > one work around > > > might be > > > > > > replace uvis_y = max(original_y, uvis_y) > > > > > > where uvis_y is the var generated by uvis. If I've done this > > > correctly, > > > then whenever -uvis- generated an unnecessary MD code for y, > > > the original > > > non-missing value for y will get plugged back in. * * 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/

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