Income Dislribul/on and Poverty in OECD Countries, Paris, 2008. In particular, the Theil index can be broken down into the sum of two factors: intra-group inequality, or the extent of dispersion of incomes within each group, and inter-group inequality which is the dispersion of average income between groups. Intra-group inequality also corresponds to the sum of the Theil indices for each group weighted by their share in the total income of that group.