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formula restriction in multinom?

2 messages · Berry, Mark R, Bert Gunter

Berry, Mark R
# 
Good Day: When I used: 
multinom(formula = Y ~ X1 + X2 + X3 + X1:X2 + X1:X3 + X3:X2 + X1^2 + X2^2 +
X3^2, data = DATASET),
I get estimates and AIC for the model containing main effects and
interactions only (no squared terms)...and FYI, all predictors are
continuous. Is this "normal" behavior? If I run this in S-Plus I get
estimates and AIC for the model containing all terms(including the squared
terms).

   Thanks,
   Mark Berry




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Bert Gunter
# 
Note: multinom() is in nnet package.

In any case, the answer is, yes, this is "normal" behavior:

x^2 <==> x:x <==> x in S language linear model notation. So if you want a
quadratic you must use I(x^2). S-Plus does this silently, but it is not
required by the language definition.

-- Bert Gunter
Genentech Non-Clinical Statistics
South San Francisco, CA
 
"The business of the statistician is to catalyze the scientific learning
process."  - George E. P. Box