Murray Pung wrote:
Is it possible to add standard error bars to the means on interaction plots?
Not with interaction.plot(), as far as I know. You might consider the
effects package by John Fox. For example:
with(ToothGrowth, interaction.plot(dose, supp, len))
ToothGrowth$dose - as.factor(ToothGrowth$dose)
TG.lm - lm(len ~ supp * dose, data = ToothGrowth)
library(effects) # loads lattice and grid packages
plot(effect(supp:dose, TG.lm))
You also could construct your own plot using the standard errors
produced by effect():
sum.df - data.frame(len = effect(supp:dose, TG.lm)$fit,
se = effect(supp:dose, TG.lm)$se,
supp = rep(c(OJ, VC), 3),
dose = rep(c(.5,1,2), each = 2))
library(Hmisc)
xYplot(Cbind(len, len - se, len + se) ~ dose | supp, data=sum.df,
type=b, ylim=c(0,30))
xYplot(Cbind(len, len - se, len + se) ~ dose, groups=supp, data=sum.df,
type=b, ylim=c(0,30))
Thanks
Murray
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