Automatically used in xls.fit()
No need to use if the raw data is not specifically desired to be achieved.
xls.prep(formula, data, dependent_var)An object of class "formula": a symbolic description of the model to be fitted.
A data.frame object.
A character which is the same as left hand side variable in specified formula.
A list object which contains a data.frame object to be modeled and character vector of independent variables.