[R] Question about factor that is numeric, in aov()

Ravi Kulkarni ravi.kulk at gmail.com
Sun May 9 14:36:18 CEST 2010

I notice something curious about how aov() treats a numeric factor:

"score" is a dependent variable and "group" is a factor in a one-way ANOVA.
But "group" contains numeric codes and is not a factor (checked with
is.factor). An ANOVA done using:

> aov(score~factor(group), data=mydata)

gives the right answers. But

> aov(score~group, data=mydata)

also produces an ANOVA table, with incorrect entries. My question is: what
exactly is R doing when I did not specify that "group" was a factor?

Ravi Kulkarni
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