How the AP did its analysis of partisan advantage in US House districts

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The Associated Press used an “efficiency gap” analysis to examine congressional elections in 2024 and other recent presidential election years.

The test — designed by Eric McGhee, a researcher at the nonpartisan Public Policy Institute of California, and Harvard Law School professor Nick Stephanopoulos — identifies states where one party is extraordinarily efficient at translating votes into victories in legislative districts. The larger the efficiency gap, the greater the advantage that either Democrats or Republicans received from the way district boundaries were drawn.

The formula also can be used to calculate how many additional seats a party won beyond what would have been expected given their share of the vote. These are referred to as “excess seats.”

The formula focuses on the vote won in each district by Democrats and Republicans, excluding independent and third-party candidates. When a major party candidate was unopposed in a particular district, the AP instead used the share of the vote won by each party’s presidential candidate in that district. The creators of the efficiency gap formula note it would be unrealistic to assume 100% of voters support a political party, so they recommend imputing a likely outcome if voters had been given a choice. Presidential votes are used because they track closely with congressional votes.

Because the efficiency gap identifies partisan advantages, it can be an indicator of intentional political gerrymandering by those who drew district lines. However, a state's voting districts also could have a partisan tilt due to other reasons, such as an abundance of like-minded voters living in the same area.