This is the name of the dataframe containing the sample data, or the frame data.
It is strictly required that auxiliary information is organised in variables named
as X1, X2, ... , Xm (there should be at least one of them) and the target variables
are denoted by Y1, Y2, ... , Yn.
In addition, in case of sample data, a variable named 'WEIGHT' must be present in the dataframe,
containing the weigths associated to each sampling unit