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

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

Subject |
st: RE: RE: RE: RE: RE: Re: Compositional data |

Date |
Wed, 5 Mar 2008 21:58:42 -0000 |

As you know, Maarten Buis is first author of -dirifit-. I am listed as second author because -dirifit- was written in the first instance as a modification of something else I wrote (which in turn was written as a modification of something else by Stephen Jenkins). But the design is Maarten's. I guess you're mixing three quite different issues on the exact 0s and 1s. 1. Substantively, exact 0s and 1s can certainly be part of genuine and informative observations. I agree completely. 2. Theoretically, I wasn't aware that the Dirichlet is general enough to include spikes in its density function. -dirifit- is aimed at fitting Dirichlet distributions. It is not intended for other distributions. 3. Practically, I guess Maarten like most authors wanted most to get a program working that was suitable for his purposes. As -dirifit- is just a wrapper for -ml-, your reference raises a very good question of whether the procedure you refer to can be implemented with -ml-. I don't know. -----Original Message----- From: owner-statalist@hsphsun2.harvard.edu [mailto:owner-statalist@hsphsun2.harvard.edu] On Behalf Of Verkuilen, Jay Sent: 05 March 2008 21:36 To: statalist@hsphsun2.harvard.edu Subject: st: RE: RE: RE: RE: Re: Compositional data Nick Cox wrote >>If three variables say x, y, z add to 1 then x + y + z = 1 defines a plane in 3-space and you can lay such a plane flat, i.e. project it onto 2-space without distortion. That, as everyone knows, is the reason you can draw a triangular plot (or whatever else it's called). What is about that which is not Euclidean? I think Euclid would have felt very much at home with that triangle.<< The issue is that the Euclidean distance between points in a triangle plot doesn't say what most people think it says in analogy to an ordinary scatterplot. That's all I meant. >>Anyway, all the alternatives I know to that stretch and shrink different parts of the space, and none is more intuitive than the original. But some can be more convenient. << The fact that compositional data are dependent due to the sum constraint makes them strange. Unordered choice data has the exact same problem. Aitchison provides some ways of dealing with the issue, but only at the expense of having to look at nasty things like log-ratios. As an aside, why did you guys kick out simplex corner observations in the Dirichlet model? These are perfectly valid observations, indeed quite possibly very informative ones since they say "I spent all my budget on X". A boundary point is a pain because the likelihood is undefined there, but the procedure described in one of Tim Fry's articles (Fry, et al, Modelling Zeroes in Microdata, Applied Economics, 2000, 33, 383-392) avoids the problem and preserves subcompositional invariance. Jay * * 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/ * * 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: RE: RE: Re: Compositional data***From:*"Nick Cox" <n.j.cox@durham.ac.uk>

**st: RE: RE: RE: RE: Re: Compositional data***From:*"Verkuilen, Jay" <JVerkuilen@gc.cuny.edu>

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