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From | Nick Cox <njcoxstata@gmail.com> |
To | statalist@hsphsun2.harvard.edu |
Subject | Re: st: xtgee for skewed data |
Date | Thu, 25 Aug 2011 15:32:25 +0100 |
No, I don't think so. Your problem as I understand it is that your response variable has granularity near 0 because the reportable values start with 0 or 0.04. This isn't a censoring or truncation problem. The bigger questions for you are whether 0 is a plausible value and also whether you want to entertain models that might predict negative values for some combination of predictors. Also, your variable sounds like a concentration so there is an upper limit too, merely that it does not bite, i.e whatever it is will never be solid beta carothen. whatever that is. Some kind of -glm, f(binomial) vce(robust) link(logit)- as very often discussed on this list might work better. Nick 2011/8/25 José Maria Pacheco de Souza <jmpsouza@usp.br>: > I am not sure whether I can include this question in the thread above, but > the subjet is related. If the interest is to run a linear regression of a > continuous variable as the response, say level of beta carothen, and for > very small values the results are zero because the equipment can only show > values equal or greater than .04, the use of -tobit- can be an statistical > alternative? > Let´s assume there is no money to buy a better device. > > Em 25/08/2011 08:15, Nick Cox escreveu: >> >> The discussion started by William Gould at >> >> >> http://blog.stata.com/2011/08/22/use-poisson-rather-than-regress-tell-a-friend/ >> >> seems relevant. You have a massive spike in the distribution. No >> transformation will much affect that, as by Murphy's theorem a spike >> maps to a spike, and in any case there would be the usual argument >> about what to do with zeros. However, (importantly different here) * * For searches and help try: * http://www.stata.com/help.cgi?search * http://www.stata.com/support/statalist/faq * http://www.ats.ucla.edu/stat/stata/