[R] different results from fligner.test

[peter dalgaard]

On Jan 10, 2012, at 21:30 , gaiarrido wrote:

> I've made fligner test with the same data, changing the orders of the
> variables, and this what i get
> 
>> fligner.test(rojos~edadysexo*zona*ano*estacion)
> 
>        Fligner-Killeen test of homogeneity of variances
> 
> data:  rojos by edadysexo by zona by ano by estacion 
> Fligner-Killeen:med chi-squared = 15.7651, df = 2, p-value = 0.0003773
> 
>> fligner.test(rojos~ano*edadysexo*zona*estacion)
> 
>        Fligner-Killeen test of homogeneity of variances
> 
> data:  rojos by ano by edadysexo by zona by estacion 
> Fligner-Killeen:med chi-squared = 86.5317, df = 3, p-value < 2.2e-16
> 
> Different results with the same variables!!! Why? i try to find an answer,
> but i really surprised

You are assuming that you can put interactions on the rhs of formulas, but nowhere in the documentation does it say that that should work. All examples have a single grouping variables. So the behavior is undefined. As far as I can see, only the _first_ variable is actually being used for grouping.  

Arguably, there is potential for better argument checking in fligner.test(), but in the meantime, I suspect that you need something like

g <- interaction(ano, edadysexo, zona, estacion)
fligner.test(rojos ~ g)

> 
> -----
> Mario Garrido Escudero
> PhD student
> Dpto. de Biología Animal, Ecología, Parasitología, Edafología y Qca. Agrícola
> Universidad de Salamanca
> --
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-- 
Peter Dalgaard, Professor,
Center for Statistics, Copenhagen Business School
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