Economics 

The Purchasing Power Parity (PPP) is a theory of exchange rate determination and a way to compare the average costs of goods and services between countries.

PPP is the rate of currency conversion that equalize the purchasing power of different currencies by eliminating the differences in price levels between countries.

The Big Mac Index is an informal way of measuring the purchasing power parity (PPP) between two currencies. It provides a view of the extent to which market exchange rates deviate from their true values.

The Robin Hood Index is conceptually one of the simplest measures of inequality used in econometrics. It is equal to the portion of the total community income that would have to be redistributed (taken from the richer half of the population and given to the poorer half) for the society to live in perfect equality.

The Phillips Curve is a graphical representation of the inverse, or negative, economic relationship between the rate of unemployment (or more precise, the rate of change in unemployment) and the percentage rate of change in money wages.

The Laffer Curve is a curve which suggests that for a given economy there is an optimal income tax level to maximize tax revenues. The Laffer Curve shows the relationship between tax rates and tax revenue collected by governments.

The Gini Coefficient is a way to measure equity and is derived from the Lorenz curve.

The Gini coefficient is defined graphically as a ratio of two surfaces involving the summation of all vertical deviations between the Lorenz curve and the perfect equality line (A) divided by the difference between the perfect equality and perfect inequality lines (A+B).

The Lorenz Curve is a graphical representation of the proportionality of a distribution. It represents a probability distribution of statistical values, and is often associated with income distribution calculations and commonly used in the analysis of inequality.