From patrick.a.oreilly at gmail.com  Wed Oct 29 12:31:20 2014
From: patrick.a.oreilly at gmail.com (Patrick O'Reilly)
Date: Wed, 29 Oct 2014 11:31:20 +0000
Subject: [Rd] terms function bug?
Message-ID: <CA+BxRLa3LO9BKxFDoJ2kfohE=T35-y2tHxC1w5qiGz40S53MRw@mail.gmail.com>

Hi,

I've noticed something strange when using the terms {stats} function.

R documentation describes the factors attribute of the terms.object as follows:

A matrix of variables by terms showing which variables appear in which terms.
The entries are 0 if the variable does not occur in the term, 1 if it does occur
and should be coded by contrasts, and 2 if it occurs and should be coded via
dummy variables for all levels (as when an intercept or lower-order term is
missing). If there are no terms other than an intercept and offsets, this is
numeric(0).
(http://stat.ethz.ch/R-manual/R-patched/library/stats/html/terms.object.html)

In the example below, I would expect Species to have a value of 2 since the
intercept is omitted. Indeed, when using model.matrix it is clear that Species
has been coded with dummy variables for all three levels.


f <- ~ -1 + Species

attr(terms(f, data=iris), "factors")
#        Species
#Species       1

levels(iris$Species)
#[1] "setosa"     "versicolor" "virginica"

colnames(model.matrix(f, iris))
#[1] "Speciessetosa"     "Speciesversicolor" "Speciesvirginica"


Is this a bug?

Many thanks in advance,

Pat



[Rd] terms function bug?
Patrick O'Reilly