Hi Christopher, i think you have the same problem like me today :) see this http://r.789695.n4.nabble.com/group-by-in-data-frame-tc3324240.html post i think you can find there the solution zem -- View this message in context: http://r.789695.n4.nabble.com/means-SD-s-and-tapply-tp3325158p3325191.html Sent from the R help mailing list archive at Nabble.com.