[R] Ancova: formula with a common intercept

[Daniel Brewer]

I have some data with two categorises plus/minus (p53) and a particular
time (Time) and the outcome is a continuous vairable (Result).  I set up
a maximum model.
ancova <- lm(Result~Time*p53)
> summary(ancova)
..
Coefficients:
             Estimate Std. Error t value Pr(>|t|)
(Intercept)   0.05919    0.55646   0.106    0.916
Time         -0.02134    0.01785  -1.195    0.241
p53plus       0.17059    0.78696   0.217    0.830
Time:p53plus  0.11887    0.02524   4.709 4.62e-05 ***
..

>From a plot of the data and the result of the linear model it looks like
the two categories share the same intercept.  How do I define this?  I
tried this:
> ancova2<-update(ancova,~.-1)
> summary(ancova2)
Call:
lm(formula = Result ~ Time + p53 + Time:p53 - 1)
..
Coefficients:
             Estimate Std. Error t value Pr(>|t|)
Time         -0.02134    0.01785  -1.195    0.241
p53minus      0.05919    0.55646   0.106    0.916
p53plus       0.22977    0.55646   0.413    0.682
Time:p53plus  0.11887    0.02524   4.709 4.62e-05 ***

But I do not think that is doing what I want.

Many thanks
-- 
**************************************************************
Daniel Brewer, Ph.D.

Institute of Cancer Research
Molecular Carcinogenesis
Email: daniel.brewer at icr.ac.uk
**************************************************************

The Institute of Cancer Research: Royal Cancer Hospital, a charitable Company Limited by Guarantee, Registered in England under Company No. 534147 with its Registered Office at 123 Old Brompton Road, London SW7 3RP.

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