Regarding the problem of getting the mean of
auth.sum corresponding to those values of datecat==1
and a separate mean corresponding to those values
of datecat==2 where auth.sum and datecat are in
data frame hcd.df, here are two solutions:

1. USE BY

  attach(hcd.df)
  by(auth.sum,datecat,mean)

or replace the second line with

  c(by(auth.sum,datecat,mean))

depending on how you like the result

2. USE LM

lm(auth.sum~factor(datecat)-1,hcd.df)$coef

---

Message-id: <Pine.LNX.4.21L1.0201092341260.5798-100000@nujoma.perrins>

I'd like to get summary statistics (really just a mean would be fine) for
a vector in a data frame, but split based on the value of another
vector. That is, I have a data frame (hcd.df) with variables datecat
(which is always 1 or 2) and auth.sum (-8..+8). I've used xtabs to get
chi-square comparisons, but what I need now is a simple mean of auth.sum
where datecat is 1 and another where datecat is 2. Thanks for any advice.
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