[R] glm.nb() giving strongly different results

[David Croll]

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