# [R] interaction() -- problem with drop?

Matthew Wiener mcw at ln.nimh.nih.gov
Thu Jun 28 21:35:54 CEST 2001

```Hi, all.

(R-1.3.0 on linux, alpha and intel; also tested on R-1.1.1 on irix.)

Below is a program that creates some random data (n, x, and y), creates a
factor out of x and y and then creates a factor z out of their interaction
(corresponding, with the default nf = 2, to quadrants, which is how I came
upon this).  It then runs an analysis of variance.

f.test.problem <-
function(n = 100, nf = 2){

t1 <- data.frame(n = rnorm(n), x = rnorm(n), y = rnorm(n))

t1\$x <- cut(t1\$x, nf, labels = 1:nf)
t1\$y <- cut(t1\$y, nf, labels = 1:nf)
t1\$z <- interaction(t1\$x, t1\$y, drop = F)

print(table(t1\$x))
print(table(t1\$y))
print(table(t1\$z))

summary(aov(n ~ z, data = t1))
}

Here's the problem:  if none of the nf * nf levels of z is empty -- that
is, if there is at least one trial taking on each value -- I get the error
"Error in model.matrix(t, data) : invalid variable type".

traceback() gives:

8: model.matrix.default(mt, mf, contrasts)
7: model.matrix(mt, mf, contrasts)
6: lm(formula = n ~ z, data = t1, singular.ok = TRUE)
5: eval(expr, envir, enclos)
4: eval(lmcall, parent.frame())
3: aov(n ~ z, data = t1)
2: summary(aov(n ~ z, data = t1))
1: f.test.problem(nf = 3)

However, if one of the levels of z is empty (which I'm checking using
table), then the analysis of variance runs!  (Easy to see if you use nf =
4 or even 3; it won't take long to get some examples that run and some
that don't.)

Creating n and z outside of a dataframe and then running aov(n~z) doesn't
help.

The problem does not arise if I do aov(n ~ x, data = t1) or
aov(n ~ y, data = t1) -- the analysis of variance runs whether there are
empty categories or not.

Finally:  if I specify drop = T in the interaction call:
t1\$z <- interaction(t1\$x, t1\$y, drop = T)
then the analysis works whether a factor actually gets dropped or
not.  That is, even when no factor is empty (and so I got an error with
drop = F), everything works.

So the problem seems to arise from something going on in interaction.  I'm
not sure what, and I'm sure someone else will see the problem faster than
I will.

Apologies in advance if I'm being dense and this is really how things
ought to work.  If not, I'll submit a formal bug report.

Matt Wiener

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```