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st: Interpretation of Curvilinear Effects


From   Christian Weiss <christian.weiss@nightberry.de>
To   statalist@hsphsun2.harvard.edu
Subject   st: Interpretation of Curvilinear Effects
Date   Tue, 9 Jun 2009 20:31:43 +0200

Dear Statalisters,

I am currently evaluating curvilinear effects of the variable X on Y
by standard OLS. To asses the curvilinearity I generated X^2 and X^3
and included these variables stepwise in the regressions. Looking at
the significance levels and the signs of the coefficients I concluded
the following:

if X and X^2 is significant:
-> X has a positive sign and X^2 has a negative sign: Bell Shaped
-> X has a negative sign and X^2 has a positive sign: U Shaped


However, the last regression analysis yielded significant X and X^2,
but both with the same sign. I am struggling how to interpret this
effect. Any help is highly appreciated! :)

Best regards
Chris
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