Re: [R] Calculation of group summaries

From: Frank E Harrell Jr <f.harrell_at_vanderbilt.edu>
Date: Wed 13 Jul 2005 - 05:57:06 EST

See http://biostat.mc.vanderbilt.edu/twiki/bin/view/Main/SasByMeansExample for one example.

Frank

Seeliger.Curt@epamail.epa.gov wrote:
> I know R has a steep learning curve, but from where I stand the slope
> looks like a sheer cliff. I'm pawing through the available docs and
> have come across examples which come close to what I want but are
> proving difficult for me to modify for my use.
>
> Calculating simple group means is fairly straight forward:
> data(PlantGrowth)
> attach(PlantGrowth)
> stack(mean(unstack(PlantGrowth)))
>
> I'd like to do something slightly more complex, using a data frame and
> groups identified by unique combinations of three id variables. There
> may be thousands of such combinations in the data. This is easy in SQL:
>
> select year,
> site_id,
> visit_no,
> mean(undercut) AS meanUndercut,
> count(undercut) AS nUndercut,
> std(undercut) AS stdUndercut
> from channelMorphology
> group by year, site_id, visit_no
> ;
>
> Reading a CSV written by SAS and selecting only records expected to have
> values is also straight forward in R, but getting those summary values
> for each site visit is currently beyond me:
>
> sub<-read.csv('c:/data/channelMorphology.csv'
> ,header=TRUE
> ,na.strings='.'
> ,sep=','
> ,strip.white=TRUE
> )
>
> undercut<-subset(sub,
> ,TRANSDIR %in% c('LF','RT')
>
> ,select=c('YEAR','SITE_ID','VISIT_NO','TRANSECT','TRANSDIR'
> ,'UNDERCUT'
> )
> ,drop=TRUE
> )
>
>
> Thanks all for your help.
> cur
> --
> Curt Seeliger, Data Ranger
> CSC, EPA/WED contractor
> 541/754-4638
> seeliger.curt@epa.gov
>
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>

-- 
Frank E Harrell Jr   Professor and Chair           School of Medicine
                      Department of Biostatistics   Vanderbilt University

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Received on Wed Jul 13 06:03:28 2005

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