The Brilliantly Simple 'Efficiency Gap' Test Can Be Used To Tell If Your State is Gerrymandered

It couldn't have come at a better time.

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Last month, the Pennsylvania Supreme Court released a brand new congressional map after that same court ruled the previous map, which was created by the Republican state legislature in 2011, was a partisan gerrymander and violated the state constitution's guarantee of "free and equal elections."

Simply put, partisan gerrymandering refers to when the party in power carves up the electoral map in its favor, and has historically been hard to prove. Politicians who gerrymander employ two main strategies when redrawing electoral maps: they concentrate as many of their opponents' votes into a handful of districts, a tactic known as "packing," and spread out the remainder of those votes thinly across multiple districts, in what's referred to as "cracking." If successful, partisan gerrymandering gives the party in charge of drawing the congressional map the majority of the power and legislative control.

Though it can be very difficult to determine if a congressional map or even single district has been the victim of partisan gerrymandering, data scientists are now coming up with a variety of accurate tests to determine if partisan gerrymandering, which effectively silences the voices of voters in a given area, has occurred.

One such test, which was used in the Pennsylvania Supreme Court case as evidence that portions of that state had been unconstitutionally gerrymandered and thus needed to be redrawn, is administered by measuring the efficiency gap. Created by Nicholas Stephanopoulos, a professor at the University of Chicago Law School, and Eric McGhee of the Public Policy Institute of California in 2015, Wired reports the efficiency gap measures the number of votes that each party "wastes" in a given election to gauge how packed and cracked its districts are. In this case, "wasted" votes are defined as those cast that do not contribute to victory.

Packing and cracking can result in weird, misshapen-looking districts, like Pennsylvania's 14th congressional district, highlighted in an Instagram post from Fair Districts PA below.

But although the whys and hows of irregular-seeming district boundaries can be difficult to untangle, the efficiency gap test gives you a single number. That number is "an indication of which side is benefiting from all of the cracking and packing and how large of an advantage they have," Stephanopoulos told The Washington Post.

Even better? The test works across both sides of the aisle — protecting both Democrats and Republicans.

What Christopher Warshaw, a political scientist at George Washington University, helped show the Pennsylvania Supreme Court, was that with the drawing of Pennsylvania's 2011 congressional map, thousands of Democratic votes were being wasted. In fact, Warshaw found the 2011 map not only gave Republicans a bigger advantage in Pennsylvania than they had before redistricting; it gave them an advantage like few the country has ever seen. "It is really one of the most extreme partisan gerrymanders in modern American history," Warshaw told Wired.

Need proof? The publication notes that while Republican candidates won only 49 percent of the congressional vote in Pennsylvania in 2012, they gained 72 percent of the seats — clear evidence of partisan gerrymandering. Furthermore, Warshaw argued this wasting of Democratic votes made it almost impossible for Democrats to see issues they support turn into federal policy, which in turn degrades trust in government and in elections.

Another method being used to prove partisan gerrymandering is known as outlier analysis, which actually came up during the oral arguments in Gill v. Whitford, a landmark United States Supreme Court case about gerrymandering in Wisconsin

Moon Duchin, an associate professor of mathematics at Tufts University who studies gerrymandering, explained to Public Radio International that outlier analysis involves using computers to generate all of the possible redistricting plans. 

"What if you knew something about those many millions of maps and you could sample from that space just the plans that are legally valid, that meet all the criteria that a plan has to meet?" she asks.

"If you can look at all those along any attribute — you could look at them by efficiency gap, simply by the number of seats for the Democrats or Republicans, by other partisan symmetry metrics, your favorite metric — what they'll do is they'll give you a bell curve and a really easy intuitive test of whether a legislature's proposed map sits in the meaty part of the bell curve, or … way out in the tail."

As gerrymandering cases continue to pop up in districts all over the country, there's no doubt many of these simple and effective tests will be put to good use.


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