poolSDTransform is a transformations that can
be applied to a matrix of data. The resulting matrix has the column-
standard deviations equal to the pool standard deviations of the
individual columns, the solution adopted by lm94superb.
poolSDTransform(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