JudgeIt allows a user to construct a model of a two-party election system over multiple election cycles, derive quantities of interest about the system through statistical estimation and simulation, and produce output summary statistics and graphical plots of those quantities. Some of the quantities of interest are based on partisan symmetry as a standard of fairness in legislative redistricting, such as partisan bias as the deviation from fairness and electoral responsiveness which indexes how party control of legislative seats responds to changes in a party's success at the polls even in a fair system. (A uniform consensus has existed in the academic literature since at least King and Browning (1987) on partisan symmetry as a standard for fairness, and even the U.S. Supreme Court now appears to agree; see Grofman and King (2007).) JudgeIt will also estimate and graph seats-votes curves, make specific vote and seat predictions for individual districts, and calculate numerous other relevant statistics.
The program can evaluate electoral systems (1) When an election already has taken place, (2) When an election has not been held yet but a new redistricting plan (or plans) has been proposed or implemented, and (3) When you wish to assess what an election would have been like if held under certain specified counterfactual conditions (such as if no minority districts had been drawn, or term limitations had prevented incumbents from running for reelection). The methods implemented in JudgeIt were developed in Gelman and King (1994).