This is a post by Ashira Menashe-Oren on varying age structures between rural and urban areas in Sub-Saharan Africa and their consequences.
The rural-urban dichotomy is the one of the most common classifications used to describe population distribution within a country. Available for many data sources, it is a simple binary measure. It is universally applicable and allows easy comparison. Demographers frequently use this rural-urban divide, especially when studying developing countries. However the terms are rarely questioned. Demographers have paid little attention to what rural and urban represent, how they are defined, and how the very definition of these terms may affect demographic measures. Intertwined with geography, demography is fundamentally a spatial science, investigating populations within regions. So shouldn’t we understand how these geographic regions affect demographic analysis?
Being a simplistic yardstick, rural or urban can measure a variety of criteria, from population density to socioeconomic status and from networking ability to administrative boundaries. Its meaning could even depend on the purpose it may serve. According to Montgomery et al. (2003), of 228 countries, half of the definitions of rural-urban are based on administrative considerations, with less than a quarter on population size and density. Thirty nine countries include socioeconomic criteria and approximately two dozen have no clear explanation of defining criteria. This causes problems for comparative studies across national borders or across time. Indeed, for decades, many have challenged this issue of different definitions, often calling for universal redefinitions or a rural-urban continuum instead.
Demographers encounter difficulties with the rural-urban dichotomy when studying urban growth and rural-to-urban migration. Urban growth depends on the interaction of two key factors – natural increase (that is the growth of the population due to the difference in deaths and births) and rural to urban migration. However, a third factor has been widely disregarded, that of the reclassification of rural areas as urban. When a settlement is redefined as urban, it usually surpasses a certain threshold in population size or density, is included in new administrative boundaries or bears fewer and fewer agricultural characteristics, depending on how each country defines “urban”. Yet, reclassification has either been ignored or lumped together with migration and little is known about the scale of reclassification, especially in developing countries.
How much of urban growth is attributed to reclassification? Using census data from Uganda for 1991 and 2002 available from the United Nations as well as data from the 2001/2 Demographic and Health Surveys, it is possible to estimate the scope of reclassification. Net age-specific migration and reclassification measured together can be estimated with the Census Survival Ratio Method (CSRM) developed by Preston (1979). It is advantageous in separating natural increase from migration to urban areas. In order to estimate migration flows without reclassification, the Demographic and Health Surveys‘ data files from Uganda for 2000/1 are used. The difference between the CSRM migration rates and these estimates provides a measure of reclassification, and its scope per age group. This is similarly done using the IPUMS 2002 census sample for comparison.
Graph 1 summarizes the different estimates of migration rates. Uganda between 1991 and 2002 has a rate of urbanization of 0.0068 (that is, the percentage difference between total population growth and urban growth), with natural increase accounting for 71.8% of urban growth and 28.2% of the urban growth attributed to migration and reclassification. The overall male migration rate (measured as the proportion of migrants to rural population) is 0.0118, compared to the 0.0105 female migration rate measured using the CSRM. However, the male age-specific migration rates are higher than females between ages 15 to 34 and then crossover to lower rates from ages 35 plus. The CSRM rates display negative values from ages 25 to 60, indicating that urban areas have higher out-migration than in-migration from rural areas. Yet, between ages 5 to 24 there are high net migration rates, adding to urban population growth. The migration rates measured using the IPUMS 2002 census sample indicate lower absolute migration rates over the past ten years that those of the CSRM method, even though they are exaggerated estimates. The rates are positive, measuring only urban in-migration. Similarly the DHS rates measure only in-migration, but solely for reproductive ages.
Findings clearly show that there are differences between the rates, indicating reclassification has a noteworthy role in the CSRM net migration rates. Indeed, reclassification accounts for nearly a quarter of CSRM measure of migration amongst the male population over 30. Amongst the female population, 30% of the net migration over age 30 is in fact the reclassification of rural areas as urban. This indicates that around 9% of urban population growth can be attributed to reclassification in those age groups.
The census survival ratio method measuring migration over-estimates net migration rates, because it includes reclassification. Thus when considering what leads to urban growth, rural-to-urban migration or natural increase, the role of migration is exaggerated if measured using the CSRM. Reclassification of areas will persist so long as urbanization continues so the CSRM estimates of migration will consistently include reclassification. Being able to separate between the changing definitions of rural areas to urban, from rural-to-urban migration will remain challenging.
This issue of reclassification of areas (and populations) and its role in urban growth is just an example of how important it is to take into consideration the definitions of rural and urban. What is meant by rural and urban? It is important to identify where their definitions create weakness in demographic research. Collaboration between geographers and demographers in creating a new set of common definitions of what is meant by rural and urban, and ultimately applying them to existing datasets and to any new research is worth deliberating.
 Uganda is a special case of changing its definitions of what is considered urban between these censuses, whereby the population threshold was doubled.
Ashira Menashe-Oren is a PhD student of Demography at the Hebrew University of Jerusalem. For any feedback contact her at firstname.lastname@example.org.