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...nce <- read.csv("http://www.ats.ucla.edu/stat/r/examples/alda/data/tolerance1.txt") tolerance.pp <- read.csv("http://www.ats.ucla.edu/stat/r/examples/alda/data/tolerance1_pp.txt") attach(tolerance.pp) # fitting the linear model by id fit <- by(tolerance.pp, id, function(bydata) fitted.values(lm(tolerance ~ time, data=bydata))) fit <- unlist(fit) # plotting the linear fit by id interaction.plot(age, id, fit, xlab="age", ylab="tolerance") # plots red for female and green for male interaction.plot(age, id, fit, xlab="age", ylab="toler...