Hi, maybe I don't understand your question correctly, but I don't see a reason why there should be a significant interaction term at all.
To the question why you regression is linear in B and C: your regressor B is linear (in B), i.e. B= 1B. your regressor C is some quadratic function of B, i.e. C=cB^2 As I see it, your C captures the quadratic form of the relationship between B and A. The relationship between C and A should therefore be linear in the regression because your C already captures the quadratic form. I.e. your C "cloaks" the significant relationship, because C is itself a quadratic function of B. Does that make any sense? Or did I understand your question incorrectly? Daniel ------------------------- cuncta stricte discussurus ------------------------- -----Ursprüngliche Nachricht----- Von: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] Im Auftrag von Rolf Turner Gesendet: Sunday, November 11, 2007 4:06 PM An: Irene Mantzouni Cc: [EMAIL PROTECTED] Betreff: Re: [R] interpretation for multiple regression 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 ###################################################################### Attention:\ This e-mail message is privileged and confid...{{dropped:9}} ______________________________________________ R-help@r-project.org mailing list https://stat.ethz.ch/mailman/listinfo/r-help PLEASE do read the posting guide http://www.R-project.org/posting-guide.html and provide commented, minimal, self-contained, reproducible code. ______________________________________________ R-help@r-project.org mailing list https://stat.ethz.ch/mailman/listinfo/r-help PLEASE do read the posting guide http://www.R-project.org/posting-guide.html and provide commented, minimal, self-contained, reproducible code.
