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Gerrymandering – the ‘Efficiency Gap’ is too unstable a measure


Much in the news lately, the U. S. Supreme Court is having a difficult time determining how much is too much partisan gerrymandering. When does a redistricting map favor a political party to an unfair degree? How do you measure unfairness?

Tony Sissons

Tony Sissons

Just looking at a map, you really can’t be sure. Some might say it’s just like pornography – you know it when you see it. But, the accepted definition of a gerrymander is: a practice intended to establish a political advantage for a particular party or group by manipulating district boundaries. Notice there is no mention of “shape.”

So, what’s a court to do? In 1986, the U. S. Supreme Court ruled that partisan gerrymandering is judiciable, but in the 32 years since, the court has not struck down a single plan on that basis.

In a movement away from geometry and geography, Nicholas O. Stephanopoulos, assistant professor of law at the University of Chicago Law School, and Eric M. McGhee, research fellow at the Public Policy Institute of California, proposed a method that measures the relative success of the major political parties in a district plan by comparing the numbers of votes “wasted” by each party. The measure is called the Efficiency Gap, and is expressed in terms of the numerical difference between both party’s wasted votes, taken as a percentage of total votes cast.

In this calculation, all the votes for the losing candidate in a district are considered wasted, as are the votes for the winning candidate in excess of the votes needed to win. Both categories of wasted votes for each party are summed for all of the districts in the plan, and then the difference between those party totals is taken as a percentage of the total number of votes cast by both parties. The party wasting the least number of votes is considered to have the advantage (for that plan for that election).

The clause in parentheses triggers my concern. I created a spreadsheet model to simplify measuring the Efficiency Gap for Arizona congressional districts since 2002. Here are the Efficiency Gaps and who they favor:


2002:            17.73 percent Republican

2004:            16.78 percent Republican

2006:            1.41 percent Republican

2008:            4.30 percent Democrat

2010:            6.70 percent Republican


2012:            11.34 percent Democrat

2014:            0.20 percent Republican

2016:            4.54 percent Republican

The figures from five elections (2002 through 2010) are two-party results on the congressional map created by the first Arizona Independent Redistricting Commission. On that map, the Efficiency Gap ranges from a 17 percent Republican advantage to a 4 percent Democratic advantage. On the same district shapes! Likewise, in districts drawn by the second redistricting commission, figures from three elections (2012 through 2016) swing from a 4 percent Republican advantage to an 11 percent Democratic advantage. Which Efficiency Gap figure is the correct one for each decade’s plan?

There is one other circumstance that casts an even greater cloud on the measure’s credibility. The results from a single highly competitive contest can really mess up the measurement. Consider: In 2014, Republican Martha McSally beat Democrat Ron Barber by 161 votes out of 219,247 votes cast between them. On the basis of that result, the Efficiency Gap for Arizona’s nine congressional districts was a 0.2 percent advantage for Republicans. But, if McSally had received 81 fewer votes and Barber 81 votes more, the Efficiency Gap on that same map would come in with a 15.5 percent Democratic advantage.

With such measurement instability, I don’t think the Efficiency Gap measurement can be considered a reliable indicator of the extent of partisan gerrymandering of Arizona election districts.

—Tony Sissons is president of Research Advisory Services, Inc., in Phoenix.


The views expressed in guest commentaries are those of the author and are not the views of the Arizona Capitol Times.

One comment

  1. Agreed. Also, the Efficiency Gap is better understood as a seats-votes equation.

    EG = (%Democratic Seats – 50%) – 2(%Democratic statewide vote share – 50%)

    Arizona has 9 congressional seats, so every time a seat flips, the EG jumps 1/9 = 11%. Your 81 vote calculation is a little off as the EG would jump 11 percentage points from that seat flipping, but you are correct in that the EG is too erratic as a measure.

    There are better measures out there. If only the courts would get on board.

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