World Urbanization Prospects, the 2011 Revision

Frequently Asked Questions

1. What is the most recent Revision of World Urbanization Prospects?

The 2011 Revision is the most recent revision of World Urbanization Prospects which are the official United Nations estimates and projections of rural and urban population as well as urban agglomerations. The 2011 Revision was released on 5 April 2012.

2. When will the next Revision of the World Urbanization Prospects be released?

The Population Division of the Department of Economic and Social Affairs of the United Nations issues a new Revision every two years. The 2013 Revision will be due in the first half of 2014.

3. How do we estimate the proportion urban and rural for each country?

The proportion of urban (and rural) population is estimated from the most recently available census or official population estimate of each country. If this estimate is only available for some period in the past, the proportion urban is extrapolated to the base year. In the 2011 Revision of the World Urbanization Prospects the base year is 2011.

4. How do we define "urban"?

We do not use our own definition of "urban" population but follow the definition that is used in each country. The definitions are generally those used by national statistical offices in carrying out the latest available census. When the definition used in the latest census was not the same as in previous censuses, the data were adjusted whenever possible so as to maintain consistency. In cases where adjustments were made, that information is included in the sources listed online. United Nations estimates and projections are based, to the extent possible, on actual enumerations. In some cases, however, it was necessary to incorporate other estimates of urban population size. When that is done, the sources of data indicate it.

5. How do we define "urban agglomeration"

The term “urban agglomeration” refers to the population contained within the contours of a contiguous territory inhabited at urban density levels without regard to administrative boundaries. It usually incorporates the population in a city or town plus that in the suburban areas lying outside of, but being adjacent to, the city boundaries. Whenever possible, data classified according to the concept of urban agglomeration are used. However, some countries do not produce data according to the concept of urban agglomeration but use instead that of metropolitan area or city proper. If possible, such data are adjusted to conform to the concept urban agglomeration. When sufficient information is not available to permit such an adjustment, data based on the concept of city proper or metropolitan area are used. The sources listed online indicate whether data were adjusted to conform to the urban agglomeration concept or whether a different concept was used.

6. What methods are used to project the urban population?

We are using a well-established and robust extrapolation method to project the urban population for the World Urbanization Prospects. The method is based on the urban-rural ratio, which we calculate for the last two empirical data points available - which are typically based on results from two censuses. The urban-rural ratio is directly related to the percentage urban. The calculations are done in two steps:

● First, we calculate the average annual rate of change in the urban-rural ratio between the last two data points. For instance, we might have census results for 2000 and 2010 for a particular country, which indicate that the urban-rural ratio in 2000 was 30/70 and the urban rural ratio in 2010 was 35/65. From these two data points we can calculate an average annual rate of change in the urban-rural ratio for the 2000 to 2010 period. This rate is equivalent to the difference between the rate of change of urban population and the rate of change of the rural population.  This rate of change is then extrapolated, assuming that the proportion urban follows a logistic path.

● In a second step we apply what we call the “world norm”. This global norm combines empirical urban-rural growth differences from 148 countries with 2 million or more inhabitants in a regression equation. The fitted regression line allows us to calculate a “hypothetical urban-rural growth difference” for each level of an initial observed percentage urban.  We can now use the most recent urban-rural growth difference of a particular country (obtained in step 1) and converge it linearly to the hypothetical urban-rural growth difference of all countries of the world (obtained in step 2) over a period of 25 years.

With this procedure, we essentially allow the urbanization process of a particular country to converge towards a worldwide observed pattern of urbanization. (In technical terms:  the urban-rural growth differences are not kept constant over the projection period, but converge towards the hypothetical urban-rural growth difference for all countries of the world with 2 million or more inhabitants).