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From |
"Quang Nguyen" <quangn@gmail.com> |

To |
statalist@hsphsun2.harvard.edu |

Subject |
Re: st: Interaction effect or running seperate models |

Date |
Thu, 27 Sep 2007 21:53:17 -1000 |

Dear Richard and Clive, Thanks so much for your great explainations. They are very helpful. With gratitude! Quang On 9/27/07, Richard Williams <Richard.A.Williams.5@nd.edu> wrote: > At 12:07 AM 9/28/2007, Quang Nguyen wrote: > >Dear Clive et al, > > > >Thanks! Yes, I certainly understand that. However, it is not my > >question. I am actually interested in the situation in which the > >effect is significant when we run seperate estimations, whereas the > >interaction term in the signle equation is not significant. Perhaps, > >an example will clarify this. > > > >Suppose: Y= a0+ a1*x + u > >Let Z is binary variable. Now, we can run two sperate OLS for Z=1 and > >Z=0.Suppose that a1 is significant for the OLS for Z=1 > > > >Alternatively, we have a single equation with interaction term: > > > >Y= m0 + m1*x + m2*z*x + v > > You also need the main effect of z in the equation: > > Y = m0 + m1*x + m2*z + m3*z*x + v > > m0 will be the intercept for group 0, m2 + m0 = intercept for group 1. > > >My question is how we can explain the case in which m2 is > >insignificant? Can we still say x has signficant effect on Y only > >given Z=1 as implied by the seperate OLS? If yes then is it better to > >run seperate OLS than single OLS w/interaction term given that the > >former can identify which sample of population impose a significant > >effect of x on y? > > You have to be careful about comparing significance of coefficients > across groups. A coefficient might be significant in one group but > not the other mostly because one has a larger sample size than the > other. You shouldn't just say that the effect is significant in one > group but not the other; a formal test of the difference across > groups should be done. > > > ------------------------------------------- > 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 > WWW: http://www.nd.edu/~rwilliam > > * > * 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/ > -- "My father gave me the greatest gift anyone could give another person, he believed in me." - Jim Valvano * * 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/

**References**:**st: Interaction effect or running seperate models***From:*"Quang Nguyen" <quangn@gmail.com>

**Re: st: Interaction effect or running seperate models***From:*"Clive Nicholas" <clivelists@googlemail.com>

**Re: st: Interaction effect or running seperate models***From:*"Quang Nguyen" <quangn@gmail.com>

**Re: st: Interaction effect or running seperate models***From:*"Clive Nicholas" <clivelists@googlemail.com>

**Re: st: Interaction effect or running seperate models***From:*"Quang Nguyen" <quangn@gmail.com>

**Re: st: Interaction effect or running seperate models***From:*Richard Williams <Richard.A.Williams.5@ND.edu>

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