[R] anova on data means

[Moshe Olshansky]

An ugly solution (but which works) would be something
like this (assuming that d is a data.frame with 4
columns named Tank, Trt, Fish, Size):

x <-1:12
for (i in 1:12) x[i] <- mean(d$Size[d$Tank == i])

Regards,
Moshe Olshansky

--- Ronaldo Reis Junior <chrysopa at gmail.com> wrote:

> Em Quinta 21 Junho 2007 16:56, Thomas Miller
> escreveu:
> > I am transitioning from SAS to R and am struggling
> with a relatively simple
> > analysis.  Have tried Venables and Ripley and
> other guides but can't find a
> > solution.
> >
> > I have an experiment with 12 tanks.  Each tank
> holds 10 fish.  The 12 tanks
> > have randomly assigned one of 4 food treatments -
> S(tarve), L(ow), M(edium)
> > and H(igh).  There are 3 reps of each treatment. 
> I collect data on size of
> > each fish at the end of the experiment.  So my
> data looks like
> >
> > Tank  Trt   Fish   Size
> > 1      S     1      3.4
> > 1      S     2      3.6
> > ....
> > 1      S    10      3.5
> > 2      L     1      3.4
> > ....
> > 12    M     10      2.1
> >
> > To do the correct test of hypothesis using anova,
> I need to calculate the
> > tank means and use those in the anova.  I have
> tried using tapply() and
> > by() functions, but when I do so I "loose" the
> treatment level because it
> > is categorical.  I have used
> > Meandat<tapply(Size,list(Tank, Trt), mean)
> >
> > But that doesn't give me a dataframe that I can
> then use to do the actual
> > aov analysis.  So what is the most efficient way
> to accomplish the analysis
> >
> > Thanks
> >
> > Tom Miller
> 
> Tom,
> 
> try the aggregate funtion. Somethink like this
> 
> meandat <- aggregate(Size,list(Tank,Trt),mean)
> 
> Inte
> Ronaldo
> --
> > Prof. Ronaldo Reis Júnior
> |  .''`. UNIMONTES/Depto. Biologia Geral/Lab. de
> Ecologia
> | : :'  : Campus Universitário Prof. Darcy Ribeiro,
> Vila Mauricéia
> | `. `'` CP: 126, CEP: 39401-089, Montes Claros - MG
> - Brasil
> |   `- Fone: (38) 3229-8187 |
> ronaldo.reis at unimontes.br | chrysopa at gmail.com
> | http://www.ppgcb.unimontes.br/ | ICQ#: 5692561 |
> LinuxUser#: 205366
> 
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