Re: [R] summing values according to a factor Dear Andy and all others who have replied: On 07/05/07, Liaw, Andy  wrote: > I guess what you want to do is compare Q1/T1 among the sections?  If you > want to compute the sum of Q1/T1 by Section, you can do something like: > > sum.by.section <- with(mydata, tapply(Q1/T1, section, sum)) > > Substitute sum with anything you want to compute. That worked perfectly.   Thanks! Enrico --  Enrico Indiogine Mathematics Education Texas A&M University hindiogine@gmail.com ______________________________________________ R-help@stat.math.ethz.ch 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.