The list is made up from information collected in recent historical books on cities.
"The numbers were drawn from the size of a city's military in peacetime and in war, household data, agricultural commerce, church records, fortification sizes, food distribution, loss of life in a disaster, and city comparisons. Chandler revised his opus in 1987, applying greater detail and further research to his numbers.
In 2003, George Modelski expanded on Chandler's work and wrote World Cities, -3,000 to 2,000. The book remains the authoritative reference on historic population today."