Understanding Partisan Redistricting Metrics

When people talk about gerrymandering, the conversation often feels abstract. We know maps can be drawn unfairly, but how do experts actually measure that? Researchers and courts rely on a set of partisan advantage metrics—statistical tools that turn election results into hard numbers showing whether a map systematically favors one party over another.

Here are some of the most important measures:

1. Efficiency Gap

The efficiency gap looks at wasted votes—ballots that don’t contribute to winning. If one party racks up lopsided wins while the other wins efficiently by smaller margins, the map may be tilted. A high efficiency gap shows that one party has been “packed” into fewer districts or “cracked” across many.

2. Mean–Median Difference

This measure compares a party’s average vote share across districts (mean) to its typical district vote share (median). If the median is much lower than the mean, it suggests gerrymandering has spread a party’s voters thin—enough to boost overall vote totals, but not enough to win districts.

3. Partisan Bias

Partisan bias asks: What would happen if both parties tied in the statewide vote? Ideally, each should win about half the seats. If one side consistently wins more, the map is biased. This metric helps reveal systematic structural advantages built into the lines.

4. Declination

The declination uses the shape of a curve drawn from district vote margins to test whether one party’s wins are clustered in a way that signals unfairness. Think of it as a geometric measure of how steeply tilted the playing field is.

5. Seats–Votes Curves

These simulate what share of seats each party would win if the statewide vote shifted up or down a few points. A fair map should respond relatively evenly to swings. A tilted map will lock in one side’s advantage even when the other party gains votes.

Why These Metrics Matter

No single measure is perfect. Courts and commissions often look at several together to understand whether a map unfairly amplifies partisan power. Using multiple tools helps guard against accusations that one side is “cherry-picking” results.

At the end of the day, these metrics are about making democracy measurable. They give us a way to move past gut feelings and toward data-driven conversations about whether voters are truly choosing their representatives—or whether representatives are choosing their voters.