R help - Nov 2007 - interpretation for multiple regression

Dear all,
 
probably this is quite clear for most of you but for me it is a headache...
 
I am regressing response A against the continuous covariate B and the
relationship is clearly quadratic.
When I add a second covariate B, the relationship becomes linear for both B and
C.
So, I expect that the interaction of B:C should also be significant, which is
not the case.
How do you interpret this??
 
Thank you!

On 10/11/2007, at 3:48 AM, Irene Mantzouni wrote:
> Dear all,
>
> probably this is quite clear for most of you but for me it is a  
> headache...
>
> I am regressing response A against the continuous covariate B and  
> the relationship is clearly quadratic.
> When I add a second covariate B, the relationship becomes linear  
> for both B and C.
> So, I expect that the interaction of B:C should also be  
> significant, which is not the case.
> How do you interpret this??

	Suppose A = a*B + b*C  and C = c*B^2.

	Then A = a*B + b*c+B^2 --- quadratic in B.

	But the original relationship, predicting A from B and C, is simply  
linear in B and C,
	no interaction term.

	Clear enuff?

		cheers,

			Rolf Turner

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