# Re: [R] abline and linearity over groups

From: Jabez Wilson <jabezwuk_at_yahoo.co.uk>
Date: Thu 04 Aug 2005 - 01:33:30 EST

But those two lines are almose identical The difference between i=0.4432, s=104.1688 and i=0.8776,s=108.1313 is almost negligible.

What I see is that abline draws a line with a v.similar slope but intercept is about 90 instead of 0.8776. Try running the example to see what I mean.

ronggui <0034058@fudan.edu.cn> wrote:>?abline and you can see
...
'reg' is a regression object which contains 'reg\$coef'. If it is of length 1 then the value is taken to be the slope of a line through the origin, otherwise, the first 2 values are taken to be the intercept and slope.
...

and
> plot(test\$l~test\$t)
> abline(lm(test\$l~test\$t))
> (lm(test\$l~test\$t))

Call:
lm(formula = test\$l ~ test\$t)

Coefficients:
(Intercept) test\$t
0.4432 104.1688

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

Call:
lm(formula = test\$l ~ test\$tF)

Coefficients:
(Intercept) test\$tF1 test\$tF2
-0.8776 108.1313 208.3376

when test\$tF is factor,these are 3 coef and the first two are used to drow the line,with Intercept = -0.8776 and slope= 108.1313 ,and abline(lm(test\$l~test\$tF)) is abline(-0.8776,108.1313)

• 2005-08-03 22:23:57 您在来信中写道：=======

>
>
>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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2005-08-03

Deparment of Sociology
Fudan University

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