Theil's Forecast Accuracy Coefficient: A Clarification

Theil's coefficient of inequality is one of the statistical forecasting evaluators frequently cited in the literature [1-7]. However, there seems to be some confusion about this coefficient which may stem from the fact that Theil himself proposed two different formulae at different times under the same name. Both are called "coefficient of inequality" and labeled with the symbol "U." In addition, there are two different interpretations possible for the formula first proposed.