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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