They further apply different schedules, according to some measure of need within the neighbourhood. In the Federal District, for example, need is assessed based on a block-level development index that combines data on social marginalisation, income and property values. Comparability across tariffs is further limited by the inclusion or exclusion of fixed fees and drainage. Despite the challenge in creating comparable indices, studies tend to find that the price for water in the Federal District is very low by comparison with the municipalities across the metropolitan area.