Dear all,
I am estimating two equations of the following (simplified) form.
Equation1: Y=b0+b1x1+e
Equation2: Y=b0+b1x1+b2x2+e
I now want to test whether adding x2 in equation two significantly
changes the coefficient of x1 (i.e., a mediation analysis a la Baron and Kenny).
It should be straightforward using suest, but I get counterintuitive results.
Both equations are estimated using NBREG (negative binomial) and here are
the relevant estimates for x1 after suest:
Suest one two
Variable� � � � � � coeff� � � � � � � � �SE� � �
� z� � � � � p(z)
One_uspapp (equation 1):
orgtype_4 |� -.2912678� �.1595529� � -1.83� �0.068
Two_uspapp (equation 2):
orgtype_4 |� �-.183896� �.1528182� � -1.20� �0.229
So, orgtype_4 (my variable x1) is not significant in equation 1 and
not significant in equation 2, and the coefficient is negative in both
equations. Yet, according to ?test? the difference between the two
coefficients is significant at 5%:
. test [one_uspapp]orgtype_4=[two_uspapp]orgtype_4;
( 1)� [one_uspapp]orgtype_4 - [two_uspapp]orgtype_4 = 0
� � � � � �chi2(� 1) =� � 4.82
� � � � �Prob > chi2 =� � 0.0281
I don?t see how the difference can be significant when both
coefficients are negative and not different from zero ? does anybody have an
explanation?
Thanks a lot for your help,
Henry
*
* For searches and help try:
* http://www.stata.com/support/faqs/res/findit.html
* http://www.stata.com/support/statalist/faq
* http://www.ats.ucla.edu/stat/stata/
