Partisan Gerrymandering: The Efficiency Gap

The US Supreme Court ruled long ago in Davis v. Bandemer (1986) that partisan gerrymandering is a justiciable issue; however, the court in Vieth v. Jubelirer (2004) failed to provide a measure that could be applied in cases involving alleged partisan gerrymandering.

In a Wisconsin case that is headed for the US Supreme Court, a three-judge US District Court panel determined that the legislature had engaged in partisan gerrymandering and adopted a test developed by Stephanopolous and McGhee in a law review article. The measure—the efficiency gap—tallies the wasted votes by each party in electoral contests for legislative seats. In a paper by Eric Petry, the method of calculating the efficiency gap is provided.  Using this method, I calculated the efficiency gap for the 52 House contests that included two major party candidates in 2016. Here are the calculations:

In the first step, Petry’s method requires the calculation of votes for each political party’s candidate in each district that met the criterion of two major party candidates and the total votes cast in those contests.

District	Rep Vote	Dem Vote	Total
11	42,481	14,276	56,757
13	55,073	14,965	70,038
23	36,501	25,501	62,002
26	39,693	28,910	68,603
27	21,536	45,718	67,254
29	44,713	28,505	73,218
33	56,802	24,199	81,001
40	8,266	23,257	31,523
41	18,924	24,863	43,787
43	31,040	19,735	50,775
44	53,997	20,411	74,408
46	10,209	37,457	47,666
47	54,785	42,635	97,420
50	24,882	43,637	68,519
53	54,741	14,256	68,997
54	28,894	23,794	52,688
64	42,158	26,288	68,446
65	34,418	26,759	61,177
66	40,368	27,240	67,608
67	41,440	29,036	70,476
70	56,684	24,057	80,741
71	44,335	10,647	54,982
78	18,030	31,185	49,215
85	35,594	23,334	58,928
89	48,341	24,861	73,202
92	35,622	24,806	60,428
93	37,002	23,987	60,989
95	11,376	35,246	46,622
96	38,991	29,434	68,425
97	39,537	27,019	66,556
98	65,348	21,547	86,895
101	15,530	30,591	46,121
102	31,595	26,208	57,803
105	23,720	23,656	47,376
107	27,086	27,992	55,078
109	11,155	53,458	64,613
111	12,520	44,918	57,438
112	31,234	23,351	54,585
113	30,501	24,795	55,296
114	37,588	27,367	64,955
115	29,987	28,939	58,926
117	27,783	29,319	57,102
118	20,831	25,632	46,463
126	35,528	23,991	59,519
134	48,192	38,958	87,150
135	32,682	26,905	59,587
136	41,643	34,077	75,720
137	8,178	18,088	26,266
144	10,745	16,287	27,032
147	11,985	43,900	55,885
149	15,840	27,613	43,453
150	47,892	27,893	75,785
	1,723,996	1,441,503	3,165,499

The second step requires the calculation of the wasted votes by each party in each district and the sum of wasted votes for each party. Note that the wasted vote total in a district is the number of votes in excess of the votes necessary to win the district (50 percent of the total vote plus one vote) for the party that won the district. For the party that lost the district, all of the votes for the party are wasted. The calculation of wasted votes by party:

District	Votes To Win	Rep Wasted Votes	Dem Wasted Votes
11	28,380	14,102	14,276
13	35,020	20,053	14,965
23	31,002	5,499	25,501
26	34,303	5,391	28,910
27	33,628	21,536	12,090
29	36,610	8,103	28,505
33	40,502	16,301	24,199
40	15,763	8,266	7,495
41	21,895	18,924	2,969
43	25,389	5,652	19,735
44	37,205	16,792	20,411
46	23,834	10,209	13,623
47	48,711	6,074	42,635
50	34,261	24,882	9,377
53	34,500	20,242	14,256
54	26,345	2,549	23,794
64	34,224	7,934	26,288
65	30,590	3,829	26,759
66	33,805	6,563	27,240
67	35,239	6,201	29,036
70	40,372	16,313	24,057
71	27,492	16,843	10,647
78	24,609	18,030	6,577
85	29,465	6,129	23,334
89	36,602	11,739	24,861
92	30,215	5,407	24,806
93	30,496	6,507	23,987
95	23,312	11,376	11,934
96	34,214	4,778	29,434
97	33,279	6,258	27,019
98	43,449	21,900	21,547
101	23,062	15,530	7,530
102	28,903	2,693	26,208
105	23,689	31	23,656
107	27,540	27,086	452
109	32,308	11,155	21,151
111	28,720	12,520	16,198
112	27,294	3,941	23,351
113	27,649	2,852	24,795
114	32,479	5,110	27,367
115	29,464	523	28,939
117	28,552	27,783	767
118	23,233	20,831	2,400
126	29,761	5,768	35,528
134	43,576	4,616	48,192
135	29,795	2,888	32,682
136	37,861	3,782	41,643
137	13,134	8,178	4,954
144	13,517	10,745	2,770
147	27,944	11,985	15,957
149	21,728	15,840	5,886
150	37,894	9,999	27,893
Total		558,230	1,058,582

The final step involves calculating the net wasted vote (Democratic Candidates’ wasted votes minus the Republican Candidates’ wasted votes) and dividing by the total number of votes cast for all candidates (3,165,499). The result is the efficiency gap, expressed as a percentage.

Rep Wasted Votes	Dem Wasted Votes	Net Wasted Votes	Efficiency Gap
558,230	1,058,582	500,352	15.81%

Consequently, the Republican Party candidates were better at converting their votes into Texas House seats. They won 15.81 percent more seats, which is 8 seats since 15.81 percent of 52 seats is 8 seats. According to Stephanopoulos and McGhee, any efficiency gap that exceeds 8 percent is unconstitutional. The efficiency gap in Texas is nearly twice that threshold.