The Polity data series is a widely used data series in political science research. The latest version, Polity IV, contains coded annual information on the level of democracy for all independent states with greater than 500,000 total population and covers the years 1800–2013. Polity's conclusions about a state's level of democracy are based on an evaluation of that state's elections for competitiveness and openness, the nature of political participation in general, and the extent of checks on executive authority. For each year and country, a "Polity Score" is determined which ranges from -10 to +10, with -10 to -6 corresponding to autocracies, -5 to 5 corresponding to anocracies, and 6 to 10 to democracies.
The Polity study was initiated in the late 1960s by Ted Robert Gurr; it is now continued by Gurr's former student, Monty G. Marshall, and sponsored by the Political Instability Task Force.
The 2002 paper "Conceptualizing and Measuring Democracy" claimed several problems with commonly used democracy rankings, including Polity, opining that the criteria used to determine "democracy" were misleadingly narrow.