Combined polity score
The Combined Polity score is computed by subtracting the AUTOCRACY score (UPP.INS.AUTO.XQ) from the DEMOCRACY score (UPP.INS.DEMO.XQ); the resulting unified polity scale ranges from +10 (strongly democratic) to -10 (strongly autocratic). Note: The POLITY score was added to the Polity IV data series in recognition of its common usage by users in quantitative research and in the overriding interest of maintaining uniformity among users in this application. The simple combination of the original DEMOC and AUTOC index values in a unitary POLITY scale, in many ways, runs contrary to the original theory stated by Eckstein and Gurr in Patterns of Authority Polity IV Project: Dataset Users’ Manual 17 (1975) and, so, should be treated and interpreted with due caution. Its primary utility is in investigative research which should be augmented by more detailed analysis. The original theory posits that autocratic and democratic authority are distinct patterns of authority, elements of which may co-exist in any particular regime context. The inclusion of this variable in the data series should not be seen as an acceptance of the counter-proposal that autocracy and democracy are alternatives or opposites in a unified authority spectrum, even though elements of this perspective may be implied in the original theory. The POLITY variable provides a convenient avenue for examining general regime effects in analyses but researchers should note that the middle of the implied POLITY “spectrum” is somewhat muddled in terms of the original theory, masking various combinations of DEMOC and AUTOC scores with the same POLITY score. Investigations involving hypotheses of varying effects of democracy and/or autocracy should employ the original Polity scheme and test DEMOC and AUTOC separately.
The Polity IV dataset covers all major, independent states in the global system (i.e., states with total population of 500,000 or more in the most recent year; currently 163 countries) over the period 1800-2009.  Changes made during the most recent update are listed in an Excel file in http://www.systemicpeace.org/inscr/inscr.htm.  For details see Monty G. Marshall, Ted Robert Gurr and Keith Jaggers “POLITY™ IV PROJECT, Political Regime Characteristics and Transitions, 1800-2009, Dataset Users’ Manual”. 
Combined Polity Score: The POLITY score is computed by subtracting the AUTOCRACY score from the DEMOCRACY score; the resulting unified polity scale ranges from +10 (strongly democratic) to -10 (strongly autocratic).