> Now I have question: what about y=b fitting? is there any model to > force or impose the ax to be zero
Rui Barradas has answered correctly for a simple lm case. You can also do lm(y~1) to obtain an estimated mean. That formulation happens to be quite useful if you are interested in a purely random-effects model* fitted using, say, lme, though you won't generally get a simple mean for unbalanced data in such a case. S Ellison ******************************************************************* This email and any attachments are confidential. Any use...{{dropped:8}} ______________________________________________ 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.