Use of a demographic base map for the presentation of areal data in epidemiology.

The purpose of this article is to offer a development ofthe demographic map as an alternative to the geographical base-map for the presentation of areal data in epidemiology. The results of geographical investigations into disease morbidity or mortality are often presented cartographically and invariably the base-nap used is the normal geographical one, showing the relevant administrative areas. The relating of disease rates to area is useful, in that the recognition of areas characterized by high or low rates may lead to clues of aetiological significance. When considering areal patterns of disease, however, the epidemiologist requires information about the size of the population at risk in the areas concerned. Sutherland (1962) drew attention to the principal deficiency of the geographical base-map in this respect. Referring to Scotland, he showed that on the normal map correct weighting could not be given to the large urban populations which occupy small areas, whilst small rural populations, often sparsely distributed over large areas, could appear to be over-represented. Thus, base-maps which would relate disease rates to the local populations at risk as well as to geographical position might prove useful epidemiological tools. Development of the demographic map, in which the area of each administrative unit is made proportional to its population whilst contiguity of geographical boundaries and the relative geographical positions of the units are maintained as far as is possible (Hollingsworth, 1966) offers interesting possibilities in this direction. The use of such maps in epidemiology is not new, Levison and Haddon (1965), for example, having used the technique for plotting cases of Wilm's tumour in New York State. To date, however, these maps have not been used to present data at the national level. 165 Sutherland (1962) developed a form of demographic diagram, using the same principle of area proportional to population, called an isodemic representation. As a basis he split Scotland into its five hospital regions and showed them separated. He thus lost geographical contiguity between the administrative units, a condition it would seem desirable to preserve. An attempt has been made in developing the agesex specific cartograms illustrated below (Figs 1, 3, and 4) to relate disease rates to both the population at risk and to geographical position in the 57 public health districts of Scotland.* A basic aim was also to try to produce cartograms similar to each other in local outline as well as in overall shape to make them readily comparable and hence of greater potential utility. Efforts to simplify their construction and final shape have also been made. To date demographic maps have presented complicated construction problems and their final appearance has often been of considerable complexity. These factors may well have obscured their advantages and contributed to their limited application so far.