The universe was also divided into four size classes, according to Henderson, so that the total urban population in each size class was approximately equal. This resulted in the following size categories:

1. Cities with populations between 100,000 and 528,000;
2. Cities with populations between 528,000 and 1,490,000;
3. Cities with populations between 1,490,000 and 4,180,000;
4. Cities with populations in excess of 4,180,000 million.

To the extent possible, the cities in each of the nine regions were sampled so that there was to be an equal number of cities in each size category.

The 2003 World Development Report and World Bank data for 1995 were used to obtain the classification of the universe of cities into four 2001 per-capita income groupings with reflection of Gross National Product (GNP) per capita in Purchasing Power Parities (PPP):

1. Cities in countries with annual GNP per-capita measured in PPP of less than $3,000;
2. Cities in countries with annual GNP per-capita measured in PPP between $3,000 and $5,200;
3. Cities in countries with annual GNP per-capita measured in PPP between $5,200 and $17,000;
4. Cities in countries with annual GNP per-capita measured in PPP of $17,000 or higher.