If you haven't already received an answer, a careful reading of

?formula

will provide it.

-- Bert

On Wed, Mar 30, 2011 at 11:42 PM, Yusuke Fukuda <Yusuke.Fukuda@nt.gov.au>wrote:

>
> Hello R experts
>
> I have two linear regressions for sexes (Male, Female, Unknown). All have a
> good correlation between body length (response variable) and head length
> (explanatory variable). I know it is not recommended, but for a good
> practical reason (the purpose of study is to find a single conversion factor
> from head length to body length), the regressions need to go through the
> origin (0 intercept).
>
> Is it possible to do ANCOVA for these regressions without intercepts? When
> I do
>
> summary(lm(body length ~ sex*head length))
>
> this will include the intercepts as below
>
>
> Coefficients:
>                       Estimate Std. Error t value Pr(>|t|)
> (Intercept)            -6.49697    1.68497  -3.856 0.000118 ***
> sexMale                -9.39340    1.97760  -4.750 2.14e-06 ***
> sexUnknown             -1.33791    2.35453  -0.568 0.569927
> head_length             7.12307    0.05503 129.443  < 2e-16 ***
> sexMale:head_length     0.31631    0.06246   5.064 4.37e-07 ***
> sexUnknown:head_length  0.19937    0.07022   2.839 0.004556 **
> ---
>
> Is there any way I can remove the intercepts so that I can simply compare
> the slopes with no intercept taken into account?
>
> Thanks for help in advance.
>
> Yusuke Fukuda
>
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>



-- 
"Men by nature long to get on to the ultimate truths, and will often be
impatient with elementary studies or fight shy of them. If it were possible
to reach the ultimate truths without the elementary studies usually prefixed
to them, these would not be preparatory studies but superfluous diversions."

-- Maimonides (1135-1204)

Bert Gunter
Genentech Nonclinical Biostatistics

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