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Re: st: Interpretation of Curvilinear Effects

From   Richard Williams <>
To   "" <>, "" <>
Subject   Re: st: Interpretation of Curvilinear Effects
Date   Tue, 09 Jun 2009 15:13:43 -0400

At 02:31 PM 6/9/2009, Christian Weiss wrote:
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

See pages 3-4 of

for pictures of various curves and info on interpreting the coefficients.

Richard Williams, Notre Dame Dept of Sociology
OFFICE: (574)631-6668, (574)631-6463
HOME:   (574)289-5227
EMAIL:  Richard.A.Williams.5@ND.Edu

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