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...ltiple variables and i'm trying to weigh average depths for fish species per year by their abundance (CPUE. I have tried the weighted.mean function but as i have two columns for the x value the lenghts differ with the w (CPUE). How do I solve this problem? So far I have tried this: data<-by(allspecies, list(allspecies$Depth, allspecies$Year), function(allspecies)weighted.mean(allspecies$CPUE)) but this doesn't seem to give the proper values, as the CPUE values are included. I also have many NA's. When i try to exclude those all my values change. Thanks for your help! much appreciated...