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From | Fabio Zona <fabio.zona@unibocconi.it> |
To | statalist@hsphsun2.harvard.edu |
Subject | Re: st: MIME-Version: 1.0 |
Date | Thu, 5 May 2011 14:07:54 +0200 (CEST) |
I agree with you, but somebody asked me to do so! And I also found it strange! ----- Messaggio originale ----- Da: "maarten buis" <maartenlbuis@googlemail.com> A: statalist@hsphsun2.harvard.edu Inviato: Giovedì, 5 maggio 2011 13:50:45 GMT +01:00 Amsterdam/Berlino/Berna/Roma/Stoccolma/Vienna Oggetto: Re: st: MIME-Version: 1.0 On Thu, May 5, 2011 at 1:05 PM, Fabio Zona wrote: > do you agree that - for an interaction effect to be significant - not only > the interaction term needs to be significant, but also the wald test of > the difference between the interaction term versus each of the two > main effects need to be significant? No I do not agree with that statement. The interaction term alone is enough. Moreover, what would the null hypothesis in such a test mean? The interaction effect tells you how much the effect of x1 changes when x2 changes, the main effect of x1 tells you how much the expected value of y changes when x1 changes when x2 == 0 (and vice versa for the main effect of x2). What would be the substantive interpretation of a null hypothesis that sets an interaction term and an main effect equal to one another? Moreover, the size and significance of the main effects trivially depend on how you centered (or whether you centered) the other variable without changing the fit of your model to the data, so the test you propose is pretty meaningless. Hope this helps, Maarten -------------------------- Maarten L. Buis Institut fuer Soziologie Universitaet Tuebingen Wilhelmstrasse 36 72074 Tuebingen Germany http://www.maartenbuis.nl -------------------------- * * For searches and help try: * http://www.stata.com/help.cgi?search * http://www.stata.com/support/statalist/faq * http://www.ats.ucla.edu/stat/stata/ * * For searches and help try: * http://www.stata.com/help.cgi?search * http://www.stata.com/support/statalist/faq * http://www.ats.ucla.edu/stat/stata/