[R] means of a column within groups of a data frame

[Daniel Malter]

Hi John, ?tapply

#generate groups, 5 with 100 obs each
group=rep(1:5,each=100)

#generate xs 2*group value+random error
x=group*2+rnorm(500,0,1)

#get mean of x for each group
#should be about Mu={2, 4, 6, 8, 10}
tapply(x,group,mean)

Cheers,
Daniel
 


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cuncta stricte discussurus
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Von: r-help-bounces at r-project.org [mailto:r-help-bounces at r-project.org] Im
Auftrag von John Sorkin
Gesendet: Wednesday, December 10, 2008 11:09 PM
An: r-help at r-project.org
Betreff: [R] means of a column within groups of a data frame

R 2.8.0
windows XP

I would like to divide the rows of data frame into five groups and then get
the mean of one column within the five groups. I have accomplished this
using the code below, but I hope there is an easier way, i.e. some function
that I can call

# create five groups.
cut(data$BMI,5)
# get mean of AAMTCARE within each of the five groups
mean(data[data[,"BMIcuts"]=="(13.3,21.9]","AAMTCARE"])
mean(data[data[,"BMIcuts"]=="(21.9,30.5]","AAMTCARE"])
mean(data[data[,"BMIcuts"]=="(30.5,39.1]","AAMTCARE"])
mean(data[data[,"BMIcuts"]=="(39.1,47.7]","AAMTCARE"])
mean(data[data[,"BMIcuts"]=="(47.7,56.3]","AAMTCARE"]) 

As always
Thank you,
John



John David Sorkin M.D., Ph.D.
Chief, Biostatistics and Informatics
University of Maryland School of Medicine Division of Gerontology Baltimore
VA Medical Center 10 North Greene Street GRECC (BT/18/GR) Baltimore, MD
21201-1524
(Phone) 410-605-7119
(Fax) 410-605-7913 (Please call phone number above prior to faxing)

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