The offset argument in the glm() quite troubles me. As below, m3 is the usage of offset that I have seen. m4 is a manually calculated analog. But the result obtained is completely different, with m3 giving the better performance. May I know why they are different and which one is correct?
standard=(ratepy/pop)*100000 m3=glm(as.integer(ratepy)~rainpd+temppd+distLon+offset(I(log(pop/1e5))), family=poisson, data=suit) m4=glm(as.integer(standard)~rainpd+temppd+distLon, family=poisson, data=suit)