Schmitter, Philippe C, and Terry Lynn Karl. "What Democracy Is. . . and Is Not." Journal of Democracy 2, no. 3 (1991): 75-88. https://dx.doi.org/10.1353/jod.1991.0033.
Democracy data: how sources differ and when to use which one There are many ways to classify and measure political systems. What approaches do different sources take? And when is which approach best?
For a discussion of two specific datasets:
International IDEA’s Democracy Assessment team produces the work that makes up the Global State of Democracy (GSoD) Initiative. The Initiative’s primary knowledge products are the annual Global State of Democracy (GSoD) Report, Democracy Tracker and the Global State of Democracy (GSoD) Indices. The Initiative also includes regional partnership networks and research and advocacy related to Sustainable Development Goal 16.