Choropleth map

A choropleth map shades predefined areas, such as districts, postcodes or census cells, according to the value of a statistic in each one. Colour carries the number, so the reader sees the pattern before reading a single label.

The important rule is that a choropleth should show a rate or a density, not a raw count. A map of the number of residents per district mostly shows which districts are large. A map of residents per square kilometre shows something about the places themselves. The same applies to any count: divide it by population, by area or by households before you map it.

The second choice that changes what the reader sees is classification. The same data split into equal intervals, quantiles or natural breaks produces three maps that tell three different stories, all of them technically correct. Say which method you used, and pick the one that matches the question rather than the one that looks strongest.

Quantile classification is one of those three methods: the data is split into groups that each hold an equal number of areas.