perc_diff               Calculate percentile differences from an
                        ordered categorical variable and a continuous
                        variable.
perc_dist               Calculate a distribution of percentiles from an
                        ordered categorical variable and a continuous
                        variable.
pisa_2006               Mathematics test scores of Spain, Germany and
                        Estonia in the PISA 2006 test
pisa_2012               Mathematics test scores of Spain, Germany and
                        Estonia in the PISA 2012 test
