On 4/10/2007, at 9:48 AM, Leeds, Mark (IED) wrote:
> When one is doing simple regression and needs to force a zero
> intercept
> ( for whatever reason. I realize it's a controversial issue ),
> then subtracting the means of the left hand side and the right hand
> side
> from themselves does the trick. Does anyone know if there is a
> similar trick when the RHS has two variables ? Thanks.
You are suffering from a bit of confusion here. There is a great
difference
between forcing the intercept to be 0 as in y ~ x1 + x2 - 1, and
transforming
(centering) the variables involved so that the intercept ***is*** 0.
The latter fits
exactly the same model as if you fitted it using the original
variables (except
that it is generally a bit more numerically stable). The former is
what is
``controversial'' (i.e. likely to be misguided).
cheers,
Rolf Turner
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