>From what you tell us it is impossible even to see if there is a problem, let >alone what it might be if there is one. There are all kinds of reasons why >intercepts may change and it is only unexpected if you do not fully understand >what the intercept parameter really is. For example, if you change a >predictor variable to have a different centre, x -> x-c, you will not change >the regression coefficient with respect to x, but by varying c you can make >the intercept anything you like. Literally. Anything. And this is nothing >whatever to do with glm.nb, it applies equally to glm, lm, aov, ...
I can console you on one point, though. glm.nb does not use a stochastic algorithm, and so no random numbers are involved. So unless you are generating fake data, the random number generator should play no part. Bill Venables http://www.cmis.csiro.au/bill.venables/ -----Original Message----- From: r-help-boun...@r-project.org [mailto:r-help-boun...@r-project.org] On Behalf Of David Croll Sent: Wednesday, 25 March 2009 12:36 PM To: r-help@r-project.org Subject: [R] glm.nb() giving strongly different results Dear colleagues, I have performed several dozens of glm.nb(response ~ variable) analyses weeks ago, and when I looked through the results today I saw that many of the results have quite different intercept values despite the response part remained the same. I'm quite sure I did same kind of analysis when the intercept values were around consistently around 2.2 and when they were above 3. When I repeated the analyses today, the intercept values were normal, they were between 2.1 to 2.3 instead of being above 3. I'm standing in front of a puzzle... they surely aren't glm() results, for they would give intercept values well above 9. Is there anything like a set.seed() thing that could have changed some properties inside R? On a second look, I discovered that the init.theta value is much lower in those analyses I have to perform again. Does anybody have a clue to this problem? It isn't that important that I have an answer (because I simply have to repeat the analyses), but still... David ______________________________________________ R-help@r-project.org mailing list https://stat.ethz.ch/mailman/listinfo/r-help PLEASE do read the posting guide http://www.R-project.org/posting-guide.html and provide commented, minimal, self-contained, reproducible code. ______________________________________________ R-help@r-project.org mailing list https://stat.ethz.ch/mailman/listinfo/r-help PLEASE do read the posting guide http://www.R-project.org/posting-guide.html and provide commented, minimal, self-contained, reproducible code.
