[R] Bartlett' test

[Peter Ehlers]

Silvano wrote:
> Hi,
> 
> I have an experiment with 5 treatments, of which 2 with 10 repetitions 
> and 2 with 7 replications.
> I conducted the test of Bartlett step-by-step and compared with the 
> value calculated directly by the R and the values are different.
> Anyone know tell me why?
> 
Yes. Is mean(tapply(valor,trat,var)) the pooled variance estimate?
Maybe you should look at the code of bartlett.test.default.

  -Peter Ehlers

> 
> #-------------------
> 
> n = length(trat)
> I = 4
> 
> (M = 2.3026*((n-I)*log10(mean(tapply(valor,trat,var))) - 
> (9*log10(vari[1])+9*log10(vari[2])+6*log10(vari[3])+6*log10(vari[4]))))
> (C = 1 + 1/(3*(4-1))*((1/9 + 1/9 + 1/6 + 1/6) - 1/(9+9+6+6)))
> (B = M/C)
> 
>> B
> 1.748670
> 
>> qchisq(.05, 3, lower.tail=F)    # Valor tabelado
> [1] 7.814728
> 
> #-----------------------------------------
> # Teste de Bartlett de forma direta no R -
> #-----------------------------------------
> 
> bartlett.test(valor,trat)
> 
>        Bartlett test of homogeneity of variances
> 
> data:  valor and trat
> Bartlett's K-squared = 0.7845, df = 3, p-value =
> 0.8532
> 
> Thanks,
> 
> --------------------------------------
> Silvano Cesar da Costa
> Departamento de Estatística
> Universidade Estadual de Londrina
> Fone: 3371-4346
> 
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