R help - Aug 2005 - abline and linearity over groups

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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>?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 伳侜佋佢伬伌佇伵佒佇佇伌伒伬仯伜======
>
>
>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

Blog:http://sociology.yculblog.com