Number of nations 1800–2003 scoring 8 or higher on the Polity IV scale, a measure of democracyWorld map showing findings from the Polity IV data series report for 2017
The data series has been criticized for its methodology, Americentrism, and connections to the CIA. Seva Gunitsky, an assistant professor at the University of Toronto, stated that the data series was appropriate "for research that examines constraints on governing elites, but not for studying the expansion of suffrage over the nineteenth century".
^Casper, Gretchen; Tufis, Claudiu (2003). "Correlation Versus Interchangeability: the Limited Robustness of Empirical Finding on Democracy Using Highly Correlated Data Sets". Political Analysis. 11 (2): 196–203. doi:10.1093/pan/mpg009.