On May 9, 2010, at 8:36 AM, Ravi Kulkarni wrote:


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?

Since you have not shown us the table we can only guess. My guess: It is treating that variable as continuous and estimating a single parameter. That may or may not be interpretable. If those codes have a meaningful order and scale, you may be getting what is sometimes called a trend test. If they are arbitrary, then the result is very probably nonsense.


Ravi Kulkarni
--


David Winsemius, MD
West Hartford, CT

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