a scalar with the column number corresponding the observed data for
the variable (the variable quality). Alternativelly can be a string with the
name of the quality variable.
sample.index
a scalar with the column number corresponding the index each
group (sample).
covar.index
optional. A scalar or numeric vector with the column number(s)
corresponding to the covariate(s). Alternativelly can be a character vector with
the names of the covariates.