High Quality Content by WIKIPEDIA articles! The Theil index, derived by econometrician Henri Theil, is a statistic used to measure economic inequality. The Theil index is derived from Shannon's measure of information entropy. Letting T be the Theil index and S be Shannon's information entropy measure, Shannon derived his entropy measure in terms of the probability of an event occurring. This can be interpreted in the Theil index as the probability a dollar drawn at random from the population came from a specific individual. This is the same as the first term, the individual's share of aggregate income.