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st: help for ovtest


From   rghuang <0034058@fudan.edu.cn>
To   "statalist@hsphsun2.harvard.edu" <statalist@hsphsun2.harvard.edu>
Subject   st: help for ovtest
Date   Tue, 23 Mar 2004 20:25:40 +0800

hi,all.
 in some relative books,i read about Ramsey  regression specification .it says as follows:
if .reg y x1 x2  ,and the fitted value is yhat,then we can regress y with x1 x2 yhat^2 yhat^3,and test if the coef of yhat^2=0 and yhat^3=0 at the same time.

so if i do
. reg  price mpg rep78
.ovtest

Ramsey RESET test using powers of the fitted values of price
       Ho:  model has no omitted variables
                  F(3, 63) =      1.47
                  Prob > F =      0.2318

and then
. predict y
.gen y2=y^2
.gen y3=y^3
.qui reg price mpg rep78 y2 y3
.test y2 y3
 ( 1)  y2 = 0
 ( 2)  y3 = 0
       Constraint 2 dropped

       F(  1,    64) =    0.33
            Prob > F =    0.5661
as we can see,the two f-value is different.so it is obvious that stata use a different formula.
and anyone can explain whow it works?

thank you!


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