[R] length() misbehaving?

[David Parkhurst]

I'm having a weird problem with length(), in R1.6.1 under windows2000.  I have a
dataframe called byyr, with ten columns, the first of which is named cnd95.
summary(byyr) shows that byyr$cnd95 contains the factor level "tr" 66 times.  Also,
when I enter byyr$cnd95 at the command line, I can count 66 "tr" elements in the
resulting vector.  However, when I enter

n95trt <- length(byyr$cnd95[byyr$cnd95=="tr"])
n95trt

the result is 68!  Any ideas why this is happening, and how I can fix the miscount?
(That column also contains 69 entries of "c", and (relevantly?) two NA's.)

Thanks for any help.

Dave Parkhurst