Brian Dickerson

Detroit Free Press Columnist

Plaintiffs who successfully challenged the constitutionality of Wisconsin’s legislative boundaries quantified the partisan bias in the state’s political map using a formula that compares the number of “wasted votes” cast for candidates of each party.

A party’s wasted-vote total is the sum of:

A) The number of ballots cast for the party’s losing candidates, and

B) The number of ballots the party’s victorious candidates received above and beyond the total each candidate needed to win.

The efficiency gap is the difference between the number of wasted votes cast for each party’s candidates in a given legislative race, divided by the total number of ballots cast.

The bigger the efficiency gap, critics of partisan gerrymandering argue, the more the political map has been rigged to one party’s advantage.

Read more:

How to make every vote count

Redistricting debate: Creepy lizard or compact lines?

Citizens can lead effort to fix district map

What is gerrymandering?

A case in point

Consider a map that carves a state of 1,000 people split evenly between the Republican Party (500 voters) and the Democratic Party (500 voters) into 10 legislative districts encompassing 100 voters each.

A map drawn to assure that Republicans and Democrats each won half the districts by a 51-49 margin would waste the same number of each party’s votes (245 in all, or 49 votes cast for the losing candidate in each of the five districts the party lost), yielding an efficiency gap of zero.

But suppose the state’s 1,000 voters were divided among one district in which Democrats outnumbered Republicans 95-5, and nine districts in which Republicans outnumbered Democrats 55-45. In that scenario, Democrats would waste 450 votes in every election (45 in the district they won, and 45 more in each of the nine districts they lost), while Republicans wasted only 50.

The difference in the number of wasted votes (450-50) divided by the total number of voters (1,000) would yield an efficiency gap of 40%.

A growing partisan advantage

The researchers who devised the formula — University of Chicago law professor Nicholas Stephanopoulos and Eric McGhee, a fellow at the Public Policy Institute of California — analyzed the results of thousands of congressional and legislative elections in the four decades between 1972 and 2012.

They found that the efficiency gaps in those elections, initially negligible, soared after political mapmakers began applying new software tools that allowed them to predict voting behavior with far greater precision.

“The efficiency gaps of today’s most egregious (redistricting) plans dwarf those of their predecessors in earlier cycles,” the two researchers wrote in a 2015 paper.

As recently as 2012, for instance, the average efficiency gap in state legislative elections was just over 6%. But in Wisconsin’s 2012 election, Democrats were on the losing end of a 13% gap.

The Democrats’ disadvantage in Michigan is even more pronounced. In the state’s 2014-16 election cycle, according to an analysis by the Center for Michigan's Bridge Magazine, the efficiency gap favoring Republicans ranged from 10.1% in elections for the state House of Representatives to 22.8% in elections for the state Senate.