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
______________________________________________
R-help@r-project.org mailing list
https://stat.ethz.ch/mailman/listinfo/r-help
PLEASE do read the posting guide http://www.R-project.org/posting-guide.html
and provide commented, minimal, self-contained, reproducible code.
