Hi Markus,
>[...] a model with two covariates. this
> doesn't look right to me, because both variables (pain and anxiety) seems
> to be regarded as equal and not one as the one which i want to ask the
> correlation and the other as the variable i want to control for.
I think this is simply a matter of using a contrast that tests the one
you are interested in (e.g. zeros over all other columns, with a 1
corresponding to this column), while ignoring the other. Simply having
the other in the design matrix will control for its effect.
Hope that helps,
Ged.
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