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Re: st: Intepretation of interaction terms

From   Maarten Buis <>
Subject   Re: st: Intepretation of interaction terms
Date   Mon, 23 May 2011 14:51:45 +0200

On Mon, May 23, 2011 at 2:33 PM, lreine ycenna <> wrote:
> If I understood correctly, you saying that the interpretation of a, b,
> and c are the same both  in (1) and (2)?
> e.g. a is the effect of a, when the other variables =0. b is the
> effect of b, when others =0. axb is the effect of both a and b when c
> =0...etc.

No, the coefficient of b is the effect of b when a = 0 (regardles of
the value the variable c takes). The the coefficient of c folows a
similar rule. But the coefficient of a is the effect of a when both b
and c are 0. Do not try to turn these into general rules. The only way
to interpret more complicated interaction effects is to write them
out. Do not be tempted to create rules or do this without the aid of
writing your equation down and deriving the effects, that will always
go wrong. In my previous post I gave examples on how to write your
equations down and derive the effects. The mathematics is not hard, it
is just a matter of proper "book keeping", that is what you need the
paper and pen for.

Hope this helps,

Maarten L. Buis
Institut fuer Soziologie
Universitaet Tuebingen
Wilhelmstrasse 36
72074 Tuebingen

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