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Re: st: interactions and subsets


From   Richard Williams <Richard.A.Williams.5@ND.edu>
To   statalist@hsphsun2.harvard.edu, statalist@hsphsun2.harvard.edu
Subject   Re: st: interactions and subsets
Date   Mon, 28 Jul 2008 18:25:51 -0500

At 05:08 PM 7/28/2008, Carlos Rodriguez wrote:
Dear all,

I am working with TSCS data in STATA8 and would appreciate some advice
on this issue.
I have run some regressions with an interaction between trade and a
dummy for late industrializers.  The interaction was not significant.
However, I reran the analysis dividing my dataset into two groups:
early industrializers and late industrialzers.  It turns out that
trade iIS significant in the set of late industrializers.  Why would
the interaction between trade and late industrializers in the whole
sample fail to achieve significance whereas trade comes up as highly
significant when the regression is run separtely on the group of
countries that are late indutrialers?  Can the difference be put down
to the dissimialr Ns? Moreover, which result shall I "believe"?
It can happen. Suppose the effect is .5 in both groups, and that one group has 100 cases and the other has 10,000 cases. In this case, the interaction would be zero, and the effect could well be significant in the big group and not be significant in the small one.

The mistake would be to eyeball results across group, note that an effect was significant in one but not the other, and conclude that the var was important in one group but not the other. It may well be that the effects do not significantly differ across groups, and that differences in significance levels reflect differences in sample size more than they reflect real differences across groups.


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Richard Williams, Notre Dame Dept of Sociology
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EMAIL: Richard.A.Williams.5@ND.Edu
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