twoStepTransform is a transformation that can
be applied to a matrix of data. The resulting matrix is both
subject-centered and bias corrected, a technique called
the CM technique b12,c05,m08superb
twoStepTransform(dta, variables)a data.frame of the same form as dta with the variables transformed.
This function is useful when passed to the argument preprocessfct of superbPlot()
where it performs a modification of the data matrix.
a data.frame containing the data in wide format;
a vector of column names on which the transformation will be applied. the remaining columns will be left unchanged