| perc_diff | Calculate percentile differences from an ordered categorical variable and a continuous variable. |
| perc_diff_df | 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 |