[R] weighting means

[Frank E Harrell Jr]

On Wed, 21 Aug 2002 12:10:44 -0400
kjetil halvorsen <kjetilh at umsanet.edu.bo> wrote:

> What about:
> 
> > ?weighted.mean
> > ?tapply
> > Q8 <- rnorm(100)
> > Faculty <- factor(c(rep(1,50), rep(2,50)))
> > w <- 1:100
> > tapply(Q8, Faculty, weighted.mean, w)
>          1          2 
> -0.0976302 -0.1457895 
> 
> 
> Kjetil Halvorsen
> 
> Tim Wilson wrote:
> > 
> > Hi everyone,
> > 
> > I've got a dataframe called 'faculty'. I want to do a weighted mean on
> > the column called 'Q8' weighted by the contents of column 'CETP'. In
> > addition, I need to operate on the result of splitting 'faculty'
> > according to the contents of a column 'FACULTY'. For example
> > 
> > > lapply(split(faculty$Q8, faculty$FACULTY), mean)
> > $"1"
> > [1] 2.2
> > 
> > $"2"
> > [1] 1.888889
> > 
> > gives the mean of 'Q8' after splitting by 'FACULTY'. When I try this
> > with weighted.mean, it doesn't work because I don't know how to specify
> > the result of the split as the argument for weighted.mean. For example,
> > 
> > > lapply(split(faculty$Q8, faculty$FACULTY), weighted.mean)
> > $"1"
> > [1] 2.2
> > 
> > $"2"
> > [1] 1.888889
> > 
> > gives the same results as above because I haven't specified the vector
> > to use for weighting. The following doesn't work at all for the obvious
> > reason that weighted.mean is not operating on the result of the split.
> > 
> > > lapply(split(faculty$Q8, faculty$FACULTY), weighted.mean(faculty$Q8,
> > faculty$CETP))
> > Error in match.fun(FUN) : not function, character, or symbol:
> > "weighted.mean(faculty$Q8, faculty$CETP)"
> > >
> > 
> > Any ideas?
> > 
> > -Tim
> > 
> > --
> > Tim Wilson      |   Visit Sibley online:   | Check out:
> > Henry Sibley HS |  http://www.isd197.org   | http://www.zope.com
> > W. St. Paul, MN |                          | http://slashdot.org
> > wilson at visi.com |  <dtml-var pithy_quote>  | http://linux.com

Also take a look at the many weighted stats functions in the Hmisc library.  Do 
library(Hmisc)
?wtd.mean

-Frank

-- 
Frank E Harrell Jr              Prof. of Biostatistics & Statistics
Div. of Biostatistics & Epidem. Dept. of Health Evaluation Sciences
U. Virginia School of Medicine  http://hesweb1.med.virginia.edu/biostat
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