Calculating concentration index with repetitive values of indicators of economic welfare

Repetitive values of the ranking indicators of economic welfare are often introduced due to incidental ties or censoring in the welfare variable, or the categorical nature of welfare variables used in numerous national surveys. In calculating concentration index (CI), assigning different fractional ranks to observations that have same values of the welfare measure leads to unstable and inconsistent CI estimates when the welfare variable is categorical or censored. In this paper, we establish an interval within which the CI estimates lie, and propose a solution, which is an extension of (Kakwani, N.C., Wagstaff, A., van Doorslaer, E., 1997. Socioeconomic inequalities in health: measurement, computation, and statistical inference. Journal of Econometrics 77, 87-103), for consistent and replicable estimates of CI when there are a substantial number of ties of the welfare indicator.