Hello,
ok, I found the problem! I now have: res_size_by_host <- tapply( selected$size, factor(selected$host), sum) instead of res_size_by_host <- tapply( selected$size, selected$host, sum) and now it works. IMHO this is strange, because selected$host is already a factor! I don't know, why this must be done... ...someone of the R-experts might know it... ...and may explain it...?! Ciao, Oliver ______________________________________________ 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.