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st: Comparaison of turning points (Inverted U curve) for the same model on different samples
From
"L.M.A. Mulotte" <[email protected]>
To
"[email protected]" <[email protected]>
Subject
st: Comparaison of turning points (Inverted U curve) for the same model on different samples
Date
Sun, 3 Jul 2011 20:06:42 +0000
Dear readers,
I have an OLS model that includes both a linear effect and the quadratic term.
I would like to do a test that compares whether the turning points obtained with two different samples are significantly different.
I would welcome any help for this test.
Here is an example showing my problem
use http://www.stata-press.com/data/r11/nlswork
gen age2=age*age
regress ln_wage age age2 birth_yr grade if south==0
------------------------------------------------------------------------------
ln_wage | Coef. Std. Err. t P>|t| [95% Conf. Interval]
-------------+----------------------------------------------------------------
age | .0676212 .0043444 15.56 0.000 .0591057 .0761368
age2 | -.0008728 .0000723 -12.07 0.000 -.0010145 -.000731
birth_yr | -.0017474 .0011531 -1.52 0.130 -.0040075 .0005128
grade | .0758 .0015348 49.39 0.000 .0727917 .0788083
_cons | -.3189213 .0826224 -3.86 0.000 -.4808699 -.1569727
------------------------------------------------------------------------------
-- Turning point is 38.74 --
regress ln_wage age age2 birth_yr grade if south==1
------------------------------------------------------------------------------
ln_wage | Coef. Std. Err. t P>|t| [95% Conf. Interval]
-------------+----------------------------------------------------------------
age | .0485752 .0049546 9.80 0.000 .0388635 .058287
age2 | -.0005538 .0000819 -6.76 0.000 -.0007144 -.0003932
birth_yr | .004853 .001272 3.82 0.000 .0023597 .0073463
grade | .077252 .0015179 50.90 0.000 .0742767 .0802273
_cons | -.531555 .0939837 -5.66 0.000 -.7157788 -.3473311
------------------------------------------------------------------------------
-- Turning point is 43.85 --
Now I want to know whether the turning point of the "south" sample (i.e. 43.85) is statistically different from the turning point of the "North" sample (i.e. 38.74)
I would be very grateful for any advice. Thanks for your consideration!
Best regards,
Louis
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