People often wonder why different sources list different populations for the same city. There are a number of reasons for the differences. One obvious one is the source for the information. These figures for 1975 come from a publication by the United Nations Centre for Human Settlements (HABITAT), An Urbanizing World: Global Report on Human Settlements 1996, (Oxford University Press, 1996), Table 4. A different organization may estimate populations differently, or even the same organization may revise its own estimates from time to time.

An equally important reason for the differences that one will encounter in different sources involves the geographic extent of the area that is considered to represent a city. Different definitions give vastly differing numbers. For example, HABITAT shows in Table 1.4 of its publication how the 1990 population of Tokyo changes with various definitions. The central city of Tokyo, defined as 23 wards, had 8,164,000 residents and covered 598 sq. km. The Tokyo prefecture (Tokyo-to) had 11,856,000 and covered 2,162 sq. km. The Greater Tokyo Metropolitan Area (including Yokohama) had 31,559,000 and covered 13,508 sq. km. The National Capital Region in Japan had 39,158,000 residents and covered 36,834 sq. km. Finally, HABITAT estimates the 1990 population of the Tokyo "Urban Agglomeration" as 25,013,000 -- giving us yet a fifth possible population to use for Tokyo in 1990! One should select the city definition that is most appropriate for the purpose at hand and use that one.