This blog post by Patricia Rogers looks at some of the risks of using choropleth maps.
Using an example from J. Samson, D. Berteaux, B. J. McGill, and M. M. Humphries, researchers at McGill University which demonstrates how the visual communication of the map can be swamped by a number of large geographic units.
Rogers, P., (2011). The risks of using choropleth maps, Genuine Evaluation. Retrieved from: http://genuineevaluation.com/the-risks-of-using-choropleth-maps/
'The risks of using choropleth maps' is referenced in: