[R] Simple summary question

[Michael A. Miller]

>>>>> "Andrew" == Andrew Perrin <andrew_perrin at unc.edu> writes:

    > 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. 

How about

  > mean( hcd.df$auth.sum[ hcd.df$datecat==1 ])
  > mean( hcd.df$auth.sum[ hcd.df$datecat==2 ])

Or you can use tapply to get means (or summaries or most anything
else) for all the levels of datacat:

  > tapply( hcd.df$auth.sum, hcd.df$datecat, mean )
  > tapply( hcd.df$auth.sum, hcd.df$datecat, summary )

Mike
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