Cancer maps of Finland: an example of small area-based mapping.

The first cancer maps of Finland based on small geographical areas, municipalities (mean population 5000 inhabitants), were drawn by the Finnish Cancer Registry in late 1950s. Since then several cancer maps based on larger administrative units, such as counties or central hospital districts, have been produced. Because of the heterogeneity of large administrative areas in terms of way of life and possible cancer risk determinants, different methods were tried to portray the geographical pattern of cancer incidence by municipality. Two major problems were encountered: (1) because of the small numbers of cases per municipality the random variation was disturbingly large when single municipality-specific rates were presented and (2) the areas of municipalities with largest populations (cities) were so small that these most important points were hardly visible on the map. After the development of computerized mapping programs, a method based on smoothed averages of municipality-based cancer incidences was selected for the Atlas of Cancer Incidence in Finland 1953-82. These maps are combinations of municipality-specific observation and the background illustrating the average cancer incidence in different parts of the country. Because of the weighting by population, a large town whose rate deviates from the level of the surrounding areas is more visible on the map than a small municipality which differs from its background in the same way. The maps show the present situation for total cancer and for the 20 most interesting specific cancer sites using a 21-color scale. In addition some comparisons are illustrated, for example geographical time trends, male/female differences, and urban/rural variations.(ABSTRACT TRUNCATED AT 250 WORDS)