Sources of population data are censuses or sample-based surveys, and are compiled regularly by national statistical offices. The results are generally published within a few years after data collection in the form of census reports or statistical yearbooks. Among the demographic variables collected in censuses, the following list contains those that are the most important for agricultural research. From this information, along with the number of births in the year prior to the census, a number of indirect measures of population dynamics can be estimated, such as birth and fertility rates. Such data are collected using a recall element in the census, but are often considered unreliable.
Migration occurs within and among countries.
This poses problems in cross-national applications where absent nationals are counted as part of the de jure population in the country of origin, but also as part of the de facto population in the receiving country. A specific aspect of migration are refugee flows due to conflict or environmental crises. These can result in very sudden movements of large numbers of people, and often result in long term displacement. Although we have come a long way since Arendal I, there are still very few comprehensive data sets available for developing countries. However, with the virtual explosion of GIS applications in a diverse range of sectors in recent past, many spatially referenced databases have been developed.