Dear Peter and Spencer, I guess that you are busy with other things now.
I just want to tell that I appreciated your help. Thanks very much! Sincerely, Tord At 15:04 2003-10-09 +0200, Peter Dalgaard BSA wrote: >Tord Snall <[EMAIL PROTECTED]> writes: > >> Null deviance: 13.1931 on 269 degrees of freedom >> Residual deviance: 9.9168 on 268 degrees of freedom >> AIC: 13.917 >. > >> BUT, note the under dispersion. I GUESS it is because I have surveyed a >> moss on marked trees at three occations (with two years in between). The >> response 1 means that the moss has disappeared, and dbh is tree diameter. >> (This corresponds to revisitng patients who has a disease, and whose weight >> is unchanged between the visits. H0: weight does not affect tha chance of >> recovery from the disease) > >Don't trust deviances as measures of dispersion with binary data! > >> Here is a version with quasibinomial: >> >. >> >> Note, no warning. >> >> I guess that this quasibinomial model is more reliable than the binomial. >> Now I can trust the SE of the Estim. too, can't I? > >No. Neither nor. > >With binary data, the deviance is purely a function of the fitted >parameters. It is the difference in -2 log L between a "perfect fit" >and the observed fit. A perfect fit has a zero prob. where the obs is >"0" and probability 1 where it is "1", and L == 1 identically in that >case. Now consider the likelihood for the "complete toss-up" i.e. >intercept and slope both equal to 0 so all probabilities are 0.5. The >likelihood in that case is 0.5^269, i.e. a constant. Take logarithms >and notice that the model deviance plus the change in deviance from >the model to the "toss-up" model is constant (2*269*log(2) to be >precise). So what appears to be a measure of residual error is >really just a measure of how far the fitted probabilities are from >0.5! > > >-- > O__ ---- Peter Dalgaard Blegdamsvej 3 > c/ /'_ --- Dept. of Biostatistics 2200 Cph. N > (*) \(*) -- University of Copenhagen Denmark Ph: (+45) 35327918 >~~~~~~~~~~ - ([EMAIL PROTECTED]) FAX: (+45) 35327907 > ----------------------------------------------------------------------- Tord Snäll Avd. f växtekologi, Evolutionsbiologiskt centrum, Uppsala universitet Dept. of Plant Ecology, Evolutionary Biology Centre, Uppsala University Villavägen 14 SE-752 36 Uppsala, Sweden Tel: 018-471 28 82 (int +46 18 471 28 82) (work) Tel: 018-25 71 33 (int +46 18 25 71 33) (home) Fax: 018-55 34 19 (int +46 18 55 34 19) (work) E-mail: [EMAIL PROTECTED] Check this: http://www.vaxtbio.uu.se/resfold/snall.htm! ______________________________________________ [EMAIL PROTECTED] mailing list https://www.stat.math.ethz.ch/mailman/listinfo/r-help
