[R] summary stats

Christos Hatzis christos at silicoinsights.com
Sun Apr 16 21:15:21 CEST 2006

You could use aggregate:

agg.mean <- aggregate(my.data, by=list(Board, Subject), FUN=mean)

The caveat is that you can only use one aggregator function at a time.  You
could rerun the same for FUN=var and FUN=length to get the additional
aggregate statustics that you need and then cbind the results:

cbind(agg.mean, agg.var, agg.n)


-----Original Message-----
From: r-help-bounces at stat.math.ethz.ch
[mailto:r-help-bounces at stat.math.ethz.ch] On Behalf Of Neil Hepburn
Sent: Sunday, April 16, 2006 2:56 PM
To: r-help at stat.math.ethz.ch
Subject: [R] summary stats

I have a data set that has student test scores along with several
categorical variables. I would like to generate a set of summary stats
(mean, variance, n) for the data grouped by school authority and by exam
topic.  I have tried the by() function but that seems to only be able to
handle one level of grouping.  In particular what I would like is something
like the following 

Board	Subject	Mean	Variance	N
board1	english	70	150		600
board2	english	66	210		510
board1	science	69	180		605
board2	science	71	220		520

and so on.

I have already generated the stats that I need using "GROUP BY" in a select
query in MySQL.  I'm just curious now about doing the same thing in R

thanks in advance,

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