Hello, Thanks. The revised 'by' I had posted works with me. An alternative, more complicated, is using split/sapply.
sapply(split(allspecies, allspecies$Year), function(x) weighted.mean(x$Depth, x$CPUE)) Rui Barradas ______________________________________________ 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.