Much (but – judging from experience – not yet enough) metaphorical ink has been spilled on this cartographic topic: Which kind of metrics lend themselves to be mapped using a choropleth map1. Or, the other way around: How should I prepare my data in order to use it in a choropleth map?
Cartographer Heather Smith has produced a video that explains the factors requiring data normalization as well as some techniques for normalization in very simple terms:
The techniques shown are not tied to any particular software. The video is part of a three-part series on designing choropleth maps with part 1 here and part 3 here.
Footnotes
i.e., a thematic map that uses different colors or shading of areas to represent data↩︎