--------------------------
Maarten L. Buis
Institut fuer Soziologie
Universitaet Tuebingen
Wilhelmstrasse 36
72074 Tuebingen
Germany
http://www.maartenbuis.nl
--------------------------
--- On Tue, 24/11/09, John Holmes wrote:
> The texts on LGM suggest that group modelling can be done
> so you are not simply estimating a single uniform effect
> on all coefficients from the grouping variable (as you
> say this is equivalent to including it as a main effect);
> but instead having two separate but simultaneously
> estimated models so that in practice each parameter has
> been given its own individual grouping variable effect.
That sounds exactly like an interaction effect between the
grouping variable and whatever other variable you are
interested in. The main effect of the grouping variable
shifts the curves uniformly up or down, and the interaction
effects change the slopes of these curves. Together, they
make sure that each group defined by the grouping variable
has its own curve, and all this is simultaneously estimated
in one model.
Hope this helps,
Maarten
--Maarten
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