The progressivity measure. Positive for progressive tax systems, and higher the
value the more progressive the system.
Arguments
income
Pre-tax income.
tax
Tax paid.
measure
Currently, only "Reynolds-Smolensky" progressivity is calculated:
$$G_Y - G_Z$$ where \(G_Y\) is the Gini coefficient of income
and \(G_X\) is the Gini coefficient of post-tax income.