[R] summary stats

John Fox jfox at mcmaster.ca
Sun Apr 16 22:43:41 CEST 2006

Dear Neil,

Coincidentally, more or less the same question was asked on r-help yesterday
and today. You can use either the by() function or aggregate(), though
you'll have to do a bit of work on the result if you want it to look just
like your example.

I hope this helps,

John Fox
Department of Sociology
McMaster University
Hamilton, Ontario
Canada L8S 4M4

> -----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 1:56 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,
> Neil
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