HronnE wrote on 09/27/2011 06:27:38 AM:
>
> Hi all
>
> This is probably a simple problem but somehow I am having much trouble
with
> finding a solution, so I seek your help!
>
> I have a data-set with continuous response variables. The explanatory
> variably is 4xpH treatments (so 8.08, 7.94, 7.81 and 7.71) so also
> continuous and not technically factorial.
>
> However I have decided to do Anova's (as well as regression) to explore
the
> effect of pH.
>
> What I don't understand is why the anova done in such a way:
>
> summary(aov(BioMass~pH))
>
> ... gives me completely different p-values if I define the pH as factor
or
> not. And what would be the correct approach?
>
> Help on this subject would be much appreciated!
>
> Hronn
If pH is a numeric variable (see ?class) then your formula is instructing
aov() to include it as a linear term with one degree of freedom, just like
lm() does if you're fitting a regression. If you want to treat pH as a
categorical variable, then you must make sure that it is included in the
model as a factor. Then you will see that it uses three degrees of
freedom.
BioMass <- rnorm(100)
pH <- sample(c(8.08, 7.94, 7.81, 7.71), size=100, replace=TRUE)
pH.f <- as.factor(pH)
summary(aov(BioMass ~ pH))
summary(lm(BioMass ~ pH))
summary(aov(BioMass ~ pH.f))
summary(lm(BioMass ~ pH.f))
Jean
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