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Re: st: comparing trends

From   Maarten Buis <>
Subject   Re: st: comparing trends
Date   Thu, 6 Sep 2012 11:31:56 +0200

On Wed, Sep 5, 2012 at 6:32 PM, Adler Scott A. wrote:
> We have run an experiment where we have subjects search for a target item among differing number of distractor items (4, 7, 9).  So we are interested in the trends in the amount of time it takes the participant to find the target as a function of the number of distractors (also called set size).  Some times, the target is present and sometimes it is not.  How do we do a comparison of the trends for target-present versus target-absent?

That sounds like an interaction effect to me, so try:

reg time i.present##c.distractors


reg time ibn.present##c.distractors

For the difference between the two see:
M.L. Buis (2012) "Stata tip 106: With or without reference", The Stata
Journal, 12(1), pp. 162-164.

Hope this helps,

Maarten L. Buis
Reichpietschufer 50
10785 Berlin

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