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st: Quadratic Function, Turning Points and Interaktion Terms

From   Lukas Borkowski <>
Subject   st: Quadratic Function, Turning Points and Interaktion Terms
Date   Tue, 8 May 2012 21:54:02 +0200

Dear all,

I work on a model that can be simplified as y = a_x1 b_x2 c_x3
My first findings suggest that the effects of x1-3 on y will change with increases in x1-3. Now, do I have to run 
reg y x1 x1sq
di _b[x1]/(2*_b[x1sq])*-1
for every x seperately or combined, thus
reg y x1 x1sq x2 x2sq x3 x3sq
in order to find the correct turning points? 

What happens when I add interaction terms, lets say for g1 and g2 for groups. 
Should I run the regression over my unrestricted equation including all interaction terms?

So my questions are: Do I check for the turning points of different variables at once or seperately and do I include my interactions terms?

Thanks for your help!

Lukas Borkowski

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