[R] anova on data means

[Peter Dalgaard]

Ronaldo Reis Junior 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)
>   

Why not just include an error term for Tank in the model?

summary(aov(Size~Trt+Error(Tank)))

 
> 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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