Aggregate calculations: Millennium Development Goal aggregation and imputation methods, weighted averages using population (WPP2012) as weight (percentage of population), total population for each regional and subregional grouping (millions). Therefore, the figures are not comparable across countries and may not be comparable over time within a country. As a result of revisions in PPP exchange rates, poverty rates for individual countries cannot be compared with poverty rates reported in earlier editions. Aggregate calculations: Millennium Development Goal aggregation and imputation methods, weighted averages using population (WPP2012) as weight (percentage of population), the total population for each regional and subregional grouping (millions).