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4. Urban Population Dynamics: Models, Measures, and Forecasts
Pages 108-154

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From page 108...
... This chapter begins by describing the features of urban population dynamics that can be seen even with the simplest of analytic mechanisms. A model of urban and rural population growth is developed to show how an initial urban and rural population distribution, when subjected to fixed demographic rates, can produce a variety of demographic outcomes: annual increases in the total urban population, the share of those increases due to migration, rates of urban growth, levels of urbanization, and their rates of change.
From page 109...
... Moreover, only "raw" estimates of city size, taken from the United Nations Demographic Yearbooks, are available for cities in the range of 100,000 to 750,000 population. The expertise of the United Nations Population Division, as expressed in its influential series World Urbanization Prospects, is focused only upon cities larger than this.
From page 110...
... Given the sizes of the rural and urban populations in a base year Ro and TO, respectively the totals Us and Rt evolve in a manner determined by four demographic rates, each of which is expressed on a per annum basis: raft the rate of natural increase in the urban population, that is, the difference between urban birth and death rates Ilr the rate of- natural increase in the rural population mama the migration rate from rural to urban areas, expressed per rural resident mll,r the migration rate from urban to rural areas, expressed per urban resident iFor more detail on the equations and their derivation, see Appendix B and United Nations (1974, 1998b)
From page 111...
... and (4.3) , the urban growth rate and the share of growth due to migration are determined by several constants the rate of urban natural increase and rates of migration to and from urban areas as well as a time-varying factor, Ut- 1 /Rt - 1, the urban/rural population balance.
From page 112...
... Flows are, by definition, short-term measures. Their empirical counterparts are found in the decompositions of intercensal urban growth that separate urban natural increase on the one hand from the sum of net migration and reclassification on the other.
From page 113...
... The more rapid population shift toward cities diminishes the relative size of the rural sector, and this in turn diminishes the relative contribution of rural migrants to city growth. The direct and feedback effects of rail work in the same direction; through both routes, a higher rate of urban natural increase reduces the migrant share of urban growth.
From page 114...
... Urban natural increase, rail, has a positive direct effect on the rate of urban growth (see equation (4.2~. The direct effect of the rural-to-urban migration rate, m,, is also positive, but its strength varies with the urban/rural population balance.
From page 115...
... 40 50 where b is the long-term urban/rural balance, that is, the value taken by U~/R~ in the limit.5 As was shown earlier, high rates of natural increase have been a defining feature of the demographic regimes of many developing countries and distinguish their urban transitions from the Western historical experience. As just noted, a higher rail produces more rapid urban growth; working indirectly through migration, a higher rural Ilr also produces more rapid urban growth.
From page 116...
... equation (B.5) , national population growth rates can be written as a weighted average of the urban and rural rates of natural increase, with the weights being the proportions of urban and rural residents in the national total.
From page 117...
... Note that when roll < for, a higher rural-to-urban migration rate mr,ll reduces the national population growth rate because it speeds the transfer of population to the urban sector where the natural rate of growth is lower. At the same time, a higher mr,~' increases the rate of urban growth.
From page 118...
... The analytic models we have been using are based on assumptions of fixed demographic rates whether these are aggregate rates of natural increase and migration in the simpler projection model, or fixed underlying schedules in the model with age structure. If such models are to help in sorting out the empirical record, i°Initially, the urban share of the combined population is 15 percent; by the end of the projection, this share has risen to about 56 percent.
From page 119...
... For example, the urban rate of natural increase, rail, may fall as a result of reductions in urban fertility. If roll falls, urban growth rates will also tend to fall, but the share of urban growth due to migration will tend to rise an outcome not predicted by a fixed-rate model.
From page 120...
... Migrant Shares as Calculated from Censuses Methodological problems bedevil all attempts to determine the relative contributions of migration, natural increase, and reclassification to urban growth. When data are lacking on migration as such, the contribution of migrants to urban growth can be estimated only imprecisely.
From page 121...
... describes some experiments in which rural mortality was assumed to be as much as 50 percent higher than urban. These experiments revealed that the decomposition of urban growth into natural increase and net migration is robust to variations in assumptions about the relative risks of mortality.
From page 122...
... i7Note that the "total" row of the table is dominated by estimates from sub-Saharan Africa and Latin America, where the greatest number of DHS surveys have been fielded. In no region of the developing world have all countries participated in the DHS program, and within regions some countries have fielded more surveys than others.
From page 123...
... 123 o .= ˘ Cal so ·_.
From page 124...
... i8See Table C-3 in Appendix C for a list of cities in the population size range from 1 to 5 million whose countries have fielded a DHS survey. The cities of over 5 million are, by region; North Africa, Cairo; sub-Saharan Africa, Lagos; Southeast Asia, Bangkok, Jakarta, and Manila; South, Central, and West Asia, Dhaka, Madras, Delhi, Calcutta, Mumbai, Karachi, and Istanbul; and Latin America, Rio de Janeiro, Sao Paulo, Mexico City, and Lima.
From page 125...
... a Number of countries with DHS survey data on migrant origin. 125 Most of the DHS surveys with data on migration also gather data on migrants' areas of origin, classified as city, town, or rural.20 Table 4-3 shows that roughly equal percentages of urban migrants come from cities and rural areas, with smaller but still substantial percentages coming from towns.
From page 126...
... The widest gaps in fertility are seen in Latin America, where the difference is on the order of 2.1 children, and in sub-Saharan Africa, where urban women are estimated to have 1.4 fewer children than rural women over a reproductive lifetime. The urban/rural differences are smaller in the other regions, although still appreciable.
From page 127...
... After all, it was not until the late nineteenth and early twentieth centuries in the West that urban mortality levels fell below rural levels (Preston and van de Walle, 1978; Preston and Haines, 1991~. The marked urban mortality advantage seen in Table 4-6 is thus a departure from the historical norm; it reflects advances in public health and scientific knowledge, and testifies to the ability of TABLE 4-6 Infant and Child Mortality, Rural and Urban Areas, by Region Child Mortalityb Infant Mortalitya DHS Surveys in Region Nc Rural North Africa 3 73.8 Sub-Saharan Africa 27 101.7 Southeast Asia 3 49.7 South, Central, West Asia 10 69.7 Latin America 13 63.3 Urban 45.8 81.0 30.4 54.2 46.9 Rural 88.5 153.6 60.6 84.6 80.7 115.9 Urban 50.3 122.0 36.8 62.2 57.0 3 TOTAL 56 82.8 63.7 87.8 a Table entries are means of Kaplan-Meier estimates of ~qO derived from 90 DHS surveys, with survey-specific estimates downweighted for countries with multiple surveys.
From page 128...
... CORE ISSUES IN DEFINITION AND MEASUREMENT The preceding section presented results by city size, which required that a linkage be made from DHS survey data on individuals to aggregate data from United Nations sources on the population sizes of cities. We have already mentioned the difficulties involved in establishing such a linkage; at this point we must assess the quality of the city population data themselves.
From page 129...
... URBAN POPULATION DYNAMICS 100 95 90 85 80 75 70 65 60 555045 40 35 30 25 10 Male ~ Female 0.06 0.04 0.02 0.00 0.02 0.04 0.06 Proportion of Urban Population FIGURE 4-5 Population pyramid for urban Brazil, 1996. 1.5— 1.4— ~ 1.3 — fir ~ 1.2— :> ~ 1.1 — a .= a Q .° 1.0 — tL 0.9 0.8 0.7 Male Urban/Rural | Female Urban/Rural b 1 1 1 1 1 1 1 1 1 1 1 1 1 0 5 10 15 20 25 30 35 40 45 50 55 60 65 Age 129 FIGURE 4-6 Urban relative to rural age composition, men and women by age, Brazil, 1996.
From page 130...
... 130 CITIES TRANSFORMED 100 95 90 85 80 75 70 65 60 55 50 45 40 35 30 25 20 15 10 5 1 I |l 1 Female| _:::::::::::::::::::1 Male 0.06 0.04 0.02 0.00 0.02 0.04 0.06 Proportion of Urban Population FIGURE 4-7 Population pyramid for urban Ghana, 1998-1999. 1.6 1.5— 1.4— 1.0— 0.9 0.e 0.7 0.6 0.5 / \ | ~ Male Urban/Rural | \ I · Female Urban/Rural | ~1 ~ ~~ I\ 1 1 1 1 1 1 1 1 1 1 1 1 1 0 5 10 15 20 25 30 35 40 45 50 55 60 65 Age FIGURE 4-8 Urban relative to rural age composition, men and women by age, Ghana, 1998-1999.
From page 131...
... As they discovered, the difficulties involved TABLE 4-8 Percentages of Urban Population in the Working Ages (15-64) , by Region and City Population Size City Population Size DHS Surveys Under 100,000 to 500,000 to 1 to Over in Region 100,000 500,000 1 million 5 million 5 million North Africa 58.2 59.6 61.3 63.6 61.8 Sub-Saharan 52.1 55.4 56.3 60.1 55.2 Africa Southeast Asia 59.6 64.3 62.7 64.3 68.7 South, Central, 56.9 59.7 61.8 62.5 62.3 West Asia Latin America 56.1 59.4 61.0 61.9 64.7 TOTAL 54.5 58.1 59.6 61.7 63.9
From page 132...
... Over the past three decades, while making such minor adjustments to its methods, the United Nations Population Division has continued to prepare estimates and projections of total urban and rural populations and of urban agglomerations, issuing reports and major updates on a biennial basis. The results are widely cited by researchers and journalists alike.
From page 133...
... If nationally determined urban criteria were to be made fully and publicly available in the major urban datasets, published alongside population size and density data, researchers would be free to study the implications of applying alternative urban criteria. Many recent censuses contain detailed information on the percentages of population living in
From page 134...
... The 1984 revisions in urban classification reduced the requirements for minimum population size and nonagricultural workforce share (Goldstein, 19904. These revisions increased both the number of cities and their populations.
From page 135...
... , presents population estimates and projections at regular 5-year intervals for urban agglomerations with populations of 750,000 and above; it includes all capital cities, irrespective of size. In preparing World Urbanization Prospects, the United Nations Population Division evidently draws its raw materials from the same population counts that are published in the Demographic Yearbook, which it then extrapolates to cover years for which census counts are unavailable.22 Curvefitting techniques akin to the URGD method are used to form city estimates and projections, involving city population growth rates (where they are available)
From page 136...
... provides further discussion of these urban concepts. In World Urbanization Prospects, the urban agglomeration is the preferred unit for which urban estimates and projections should be prepared.
From page 137...
... In these cases, the World Urbanization Prospects estimates often represent the size of the city proper; occasionally, they represent metropolitan areas rather than urban agglomerations. Examples The figures that follow illustrate some of the difficulties of interpretation that surround the United Nations' city population estimates.
From page 138...
... It is then disconcerting to discover that for Shubra-El-Khema, the World Urbanization Prospects estimates are very close to those published in the Demographic Yearbook for the city proper; a larger gap would have been expected if the World Urbanization Prospects estimates faithfully represented urban agglomerations. The United Nations (1998b: 62)
From page 139...
... _ ~ 10 . _ o Q o 5 o · Metropolitan Area; World Urbanization Prospects O Metropolitan Area; Demographic Yearbook o i.' O 0 00 A/ T ~ ~ ~ ~ 1950 1960 1970 1980 1990 2000 Year FIGURE 4-11 Sao Paulo: United Nations population estimates.
From page 140...
... 700 600 500._ City Proper; Demographic Yearbook Capitals; World Urbanization Prospects 300 200 f 1950 1960 1970 1980 1990 2000 Year FIGURE 4-13 Niamey, Niger: United Nations population estimates.
From page 141...
... Although the broad outlines of the URGD method are known from the United Nations publications, the details of its application to city populations have not been placed in the public domain. Hence, little is known about the gains that might result from more sophisticated statistical modeling.
From page 142...
... It is easy to find examples of projections at the city level that have proven to be wildly in error. For instance, United Nations projections of the population of Lagos for 2000 have fluctuated with each successive update of World Urbanization Prospects.
From page 143...
... The 2000 Mexican census recorded a population of 18.1 million for Mexico City, and Sao Paulo's population is reckoned at 17.9 million for the same year.24 Are these isolated cases that illustrate the inevitable errors of any projection, or are the United Nations projections assembled in a way that somehow tends to impart an upward bias to the projections for large cities? Because the United Nations does not place its city projection materials and methods fully in the public domain, we cannot say whether particular assumptions or data errors might produce systematic biases.
From page 144...
... in Urban Population Projections for the Year 2000, by Length of Forecast, Region, Level of Development, and Size of Country Category Region East Asia and Pacific (EAP) EAP excluding China Europe Latin America and Caribbean Middle East and North Africa South Asia Sub-Saharan Africa Organisation for Economic Cooperation and Development Other High-Income Level of Development Low Lower Middle Lower Middle excluding China Upper Middle High Size of Country 0-2 million 2-10 million 10-50 million 50+ million 50+ million excluding China MPEa MAPEb 20 yearsC 10 yearsd 5 yearse 20 years 10 years 5 years 0.039 0.184 0.140 0.198 0.133 0.272 0.218 0.267 -0.028 0.098 -0.004 0.130 0.088 0.054 -0.009 0.068 0.085 0.197 0.027 0.234 0.055 0.068 -0.024 -0.018 -0.183 -0.102 -0.056 0.231 0.069 0.256 0.099 0.037 0.128 0.089 0.008 0.060 -0.027 -0.019 0.183 0.032 0.261 -0.013 World Excluding China 0.113 0.295 0.140 0.226 0.245 0.291 0.382 0.289 0.043 0.166 0.053 0.130 0.088 0.075 0.021 0.123 0.105 0.197 0.070 0.274 0.097 0.110 0.048 0.020 0.334 0.199 0.072 0.312 0.115 0.279 0.199 0.117 0.074 0.063 0.030 0.120 0.098 0.019 0.216 0.108 0.027 0.124 0.192 0.001 0.189 0.126 0.018 0.141 0.171 0.007 0.190 0.120 0.020 0.199 0.080 0.283 0.049 0.161 0.066 0.115 0.026 0.053 0.022 0.528 0.268 0.169 0.282 0.199 0.082 0.329 0.163 0.070 0.168 0.208 0.049 0.227 0.149 0.054 0.206 0.199 0.055 0.257 0.156 0.060 NOTE: Based on 169 countries and territories whose boundaries have not changed substantially over the last 20 years.
From page 145...
... Among all cities of 750,000 or more residents whose populations were projected in 1980, the median reduction in projected city size was 15.1 percentage points. Upward revisions in the projections were far less common than downward revisions.
From page 146...
... STATISTICAL SYSTEMS FOR DISAGGREGATED DATA Evidently, the aggregate databases on city size and growth are in need of substantial repair. But the United Nations demographers cannot be charged with this task: although they have great expertise and a store of critical knowledge, they must depend on the figures contributed by national statistical agencies.
From page 147...
... These remotely sensed data alone were found to be strongly correlated with census population counts, and the use of ancilliary socioeconomic information further strengthened the correlation. Methods such as these have good potential to improve estimates of the spatial extent of city populations.
From page 148...
... Factors favoring the country's advanced use of GIS technology include its small geographic size, high-level political support for GIS initiatives (including the authority to mandate and enforce uniform standards) , outstanding technical leadership, and adequate funding (Tosta, 1997~.
From page 149...
... And India is home to a remarkable software industry. Why, then, has its use of GIS technology not progressed further?
From page 150...
... . Other Applications Elsewhere around the world, GIS technology is being applied in innovative ways to improve urban management.
From page 151...
... Conclusions The analytic models examined in this chapter highlight a point that is often overlooked: urban growth rates and the migrant shares of growth will both tend to be high when a country is in its initial stages of urbanization; both will then tend to decline as the level of urbanization rises. The linkages of urban natural increase to the rate of urbanization can also be misperceived.
From page 152...
... Unfortunately, the United Nations estimates of city size, as presented in World Urbanization Prospects, are more heterogeneous and subject to measurement error than is commonly realized. We reviewed several cases and found that only the most attentive and dogged researcher would be likely to understand the
From page 153...
... Much the same can be said of the United Nations projections, which have often proven to be so far off the mark that consideration of alternative projection methods is now badly needed. Recommendations for Urban Research Infrastructure As countries urbanize, the proliferation of cities and increases in average city sizes heighten the need for adequate urban population data.
From page 154...
... United Nations projections of the populations of large cities have displayed a tendency toward upward bias. Total urban populations have also been projected to grow at rates that, in retrospect, were much too rapid.


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