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st: Quadratic term validity


From   Rozilee Asid <rozilee@streamyx.com>
To   statalist@hsphsun2.harvard.edu
Subject   st: Quadratic term validity
Date   Thu, 11 Nov 2004 05:02:42 +0800

Dear statalister

My wage model consists of several variables and model. One of my model
consists of quadratic term of age, example
Ln-wage = alpha0 + alpha1.ln_age + alpha2.ln_exp (model 1)
Ln_wage = alpha0 + alpha1.ln_age + alpha2.ln_exp + alpha3.ln_age^2 (model 2)

My main attention is to identified whether age play its significant role in
the model. When I regress the model I found that alpha1 coefficient is
negative and insignificant and alpha3 is positive and significant. My
question is before I include the quadratic term of age variable (model 1),
the alpha1 coefficient is positive and significant.

Is it valid for me to report the finding from model 2 equations, especially
when alpha1 is negative in the model.

Hope someone could help me in this matter.

Thanks.



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