How to estimate market concentration given grouped data

A novel method is proposed of estimating market concentration for the census case in which firms are grouped into size classes and all that is known about the firms in each category is their number and aggregate size. The formula arises as a way of applying an alternative expression for the concentration ratio based upon the original and first moment size distributions of firms. The semi-parametric technique, which includes a method of interval as well as point estimation, is derived, applied and validated using actual and simulated data. It is anticipated that the estimator will be of use in competition analysis as well as in academic research.