The National Population Census carried out in July 2002 provided detailed information about the size of the population and its regional distribution in Poland. Comparisons of this information with the estimation based on current registration revealed that the population in Poland was about 400 thousands people smaller than estimated. The article discusses the sources of these differences. It is common to explain differences between the census and register data with differences in methodology used by the two data sources. The differences were expected and were supposed to be a result of unregistered emigration, which is possible to grasp only by the census. However, the analysis, especially in the regional breakdown, shows that this is only a part of the explanation. Differences in census and register data by regions can be caused by administrative changes, especially related to spatial development of cities and formation of new cities within some old ones. Analyses demonstrated remarkable overestimation and underestimation of number of people in regions according to registration in comparison to the census. The most remarkable differences were found for the greatest agglomerations - Katowice (about -70 thousands) and Warsaw ones (about +100 thousands). Considerable regional differences call into question recent studies on spatial dimensions of demographic processes. It seems to be necessary to reformulate current conclusions concerning directions of demographic development by regions and assumptions of demographic forecasts. Key words: population estimates versus the census population, regional differences in the population size
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