Presentation, communication and dissemination of vital statistics
The old adage that a picture is worth a thousand words could equally well apply to the ability of a map to summarise a large amount of data. A lot of statistical data in countries are collected for geographic subregions; this kind of spatial relationship is particularly important for vital statistics as it helps identify points of weaknesses as well as inequalities in the data. A well-designed map is easy to understand and helps people grasp a large amount of information in one glance.
Many cartographic software packages are now available at low prices or even free on the internet, and can be used with Excel data and without programming expertise. Some offer desktop global geocoding possibilities while others are country-specific and based on postal codes (Australia). However, when very small geographic units are used, the disease data for reasons of both confidentiality and stochastic variation should not be used. Additionally, it is crucial to verify that the data being displayed conform to the spatial units used in the map.
Maps, like scatter plots, are useful as tools for data verification.
Thematic statistical maps can be used to show spatial differences in completeness of registration or disease prevalence. The same advice given for charts also applies to maps – the nature of the data and the message to be conveyed very much decides the selection of the appropriate mapping technique. The most useful type of maps for vital statistics data are choropleth maps, in which each area is shaded according to the value of the particular indicator being displayed. The values in choropleth maps should always be ratios, rates or densities of some type, never absolute values. The three key rules for choropleth maps are:
- All class ranges should be unambiguous and not overlapping
- There should be no gaps between the classes
- Areas for which no data exist should be identified.
Dot density maps and proportional symbol maps are two other types that can be used for absolute values. These are best used for phenomena that change smoothly over space. However, the actual placement of the dot can cause some confusion with readers believing that the exact position of the dot is meaningful. Similarly, if there are too many dots, it can be difficult to interpret.