[R] abline and linearity over groups

From: Jabez Wilson <jabezwuk_at_yahoo.co.uk>
Date: Thu 04 Aug 2005 - 00:23:57 EST

Dear R users, please can you help me understand the behaviour of abline using function lm.

I'm trying to learn linearity over groups. So I make three groups with 10 values each:

test=data.frame(cbind(
l=c(rnorm(10,0,30),rnorm(10,100,30),rnorm(10,200,30)), t = c(rep(0,10), rep(1,10), rep(2,10))
))

when I do:

plot(test$l~test$t)
abline(lm(test$l~test$t))

the abline is a straight line through the centre of the points of each of the groups.

If, however, I factorise the groups (in order to do e.g. anova analysis) and then plot the data

test$tF=factor(test$t)
plot(test$l~test$tF)
abline(lm(test$l~test$tF))

the abline is now shifted up and to the left of where I would expect the line to go (through the centre of the points of each of the groups).

If there is a simple explanation, could someone tell me it?                   



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