In June, the Supreme Court agreed to hear its first partisan gerrymandering case in more than a decade. This case, Gill v. Whitford, involves a challenge to the district plan that Wisconsin passed for its state house after the 2010 Census. The case also involves a quantitative measure of gerrymandering — the efficiency gap — that has created a bit of a buzz. One reporter compares it to a “silver-bullet democracy theorem” and a “gerrymandering miracle drug.” Another speculates that it may be the “holy grail of election law jurisprudence.” I’m an attorney in Whitford and the co-author of an article advocating the efficiency gap, so I appreciate the attention the metric is getting. But I still think much of this interest is misplaced. The efficiency gap is, in fact, a simple and intuitive measure of gerrymandering, and I’ll explain why in a minute. But the true breakthrough in Whitford isn’t that plaintiffs have finally managed to quantify gerrymandering. Rather, it’s that they’ve used the efficiency gap (and other metrics) to analyze the Wisconsin plan in new and powerful ways. These analyses are the real story of the litigation — not the formulas that enabled them.
For more than three decades, the Supreme Court has recognized that severe partisan gerrymandering can violate the Constitution. But until Whitford, not a single federal court had struck down a map on this basis. Early litigants lost because the standard the courts used prior to 2004 was so demanding it could never be met. Since 2004, the courts haven’t even been able to agree on a test, rendering most lawsuits hopeless. However, in a 2006 case, five justices expressed interest in statistical metrics that show how a plan benefits (or handicaps) a given party. The efficiency gap is such a metric.
To grasp the efficiency gap, you have to realize that partisan gerrymandering is always carried out in one of two ways. Either a party cracks (that is, splits) the other party’s voters among many districts in which their preferred candidates lose by relatively narrow margins. Or a party packs the other side’s voters into a few districts in which their preferred candidates win by overwhelming margins.
Both cracking and packing produce wasted votes that don’t contribute to a candidate’s election. In the case of cracking, these are all of the votes cast for a losing candidate. In the case of packing, they’re all of the votes cast for a winning candidate in excess of the 50 percent (plus one) needed for victory. The efficiency gap is simply one party’s total wasted votes, minus the other party’s total wasted votes, divided by all of the votes cast in the election.