How do you tell if a district was gerrymandered?

Bizarre district shapes are the popular signal and the least reliable one, since compact districts can be drawn to entrench a party and irregular ones can result from following rivers, county lines, or Voting Rights Act obligations. The more informative measures are quantitative. The efficiency gap counts wasted votes, meaning votes for losing candidates plus surplus votes beyond what a winner needed, and compares the two parties' totals, on the theory that systematic asymmetry indicates deliberate packing and cracking. Partisan symmetry asks whether each party would win a similar number of seats if their vote shares were reversed.

The method that has become most influential in litigation is ensemble analysis. Researchers use algorithms to generate thousands or millions of alternative maps satisfying all the neutral legal criteria, equal population, contiguity, compactness, respect for political subdivisions, and then compare the enacted map's partisan outcome against that distribution. A map falling far outside the range produced by nonpartisan computer generation is difficult to explain as coincidence, and this approach has featured in state court cases in several states.

The legal landscape shapes what any of this accomplishes. The Supreme Court held in Rucho v. Common Cause in 2019 that partisan gerrymandering claims present political questions beyond federal court review, which pushed challenges to state courts applying state constitutions, where several have succeeded. Racial gerrymandering remains reviewable under the Constitution and the Voting Rights Act, though the standards have been narrowed over successive decisions. Meanwhile a growing number of states have moved redistricting to independent or bipartisan commissions, with evaluations of their results generally positive but mixed depending on how insulated from partisan appointment they actually are.