[R] indicator or deviation contrasts in log-linear modelling

maiya maja.zaloznik at gmail.com
Wed Feb 18 23:47:33 CET 2009


I am fairly new to log-linear modelling, so as opposed to trying to fit
modells, I am still trying to figure out how it actually works - hence I am
looking at the interpretation of parameters. Now it seems most people skip
this part and go directly to measuring model fit, so I am finding very few
references to actual parameters, and am of course clear on the fact that
their choice is irelevant for the actual model fit. 

But here is my question: loglin uses deviation contrasts, so the
coefficients in each term add up to zero.
Another option are indicator contrasts, where a reference category is chosen
in each term and set to zero, while the others are relative to it. My
question is if there is a log-linear command equivalent to loglin that uses
this secong "dummy coding" style of constraints (I know e.g. spss genlog
does this). 

I hope this is not to basic a question!

And if anyone is up for answeing the wider question of why log-linear
parameters are not something to be looked at - which might just be my
impression of the literature - feel free to comment!

Thanks for your help!

Maja

edit: this might just be me using the wrong terminology! if idicator and
deviation contrasts come by a different name, I would love to know it! tnx
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