[R] Forcing zero intercept in two predictor case - stat question not R question

[Rolf Turner]

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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