§ 20A-19-103 — Redistricting standards and requirements.

(1) As used in this section:
(1)(a) "Ensemble analysis" means an analysis of a proposed redistricting plan:
(1)(a)(i) that uses a sequential Monte Carlo simulation to generate an ensemble of at least 4,000 redistricting plans before culling any plan for compliance with this section;
(1)(a)(ii) that indicates whether a proposed redistricting plan shows a partisan intent by comparing the proposed plan to the ensemble by calculating:
(1)(a)(ii)(A) the ranked marginal deviation of the ensemble by:
(1)(a)(ii)(A)(I) identifying one of the two major political parties as the reference party;
(1)(a)(ii)(A)(II) for each map the simulation generates, ranking the districts from worst performance for the reference party to best performance for the reference party, using the partisan index;
(1)(a)(ii)(A)(III) at each rank, calculating the average vote share for the reference party;
(1)(a)(ii)(A)(IV) for each map the simulation generates, calculating the deviations between the reference party's performance at each rank and the average vote share at that rank;
(1)(a)(ii)(A)(V) squaring the deviations described in Subsection (1)(a)(ii)(A)(IV);
(1)(a)(ii)(A)(VI) adding together the squared deviations described in Subsection (1)(a)(ii)(A)(V) for each map;
(1)(a)(ii)(A)(VII) dividing the sum described in Subsection (1)(a)(ii)(A)(VI) by the number of districts; and
(1)(a)(ii)(A)(VIII) taking the square root of the quotient described in Subsection (1)(a)(ii)(A)(VII); and
(1)(a)(ii)(B) the ranked marginal deviation of the proposed redistricting plan by:
(1)(a)(ii)(B)(I) ranking the districts from worst performance for the reference party to best performance for the reference party, using the partisan index;
(1)(a)(ii)(B)(II) calculating the deviations between the reference party's performance at each rank and the average vote share described in Subsection (1)(a)(ii)(A)(IV);
(1)(a)(ii)(B)(III) squaring the deviations described in Subsection (1)(a)(ii)(B)(II);
(1)(a)(ii)(B)(IV) adding together the squared deviations described in Subsection (1)(a)(ii)(B)(III);
(1)(a)(ii)(B)(V) dividing the sum described in Subsection (1)(a)(ii)(B)(IV) by the number of districts; and
(1)(a)(ii)(B)(VI) taking the square root of the quotient described in Subsection (1)(a)(ii)(B)(V); and
(1)(a)(iii) for which the following results fail the ensemble analysis:
(1)(a)(iii)(A) a result described in Subsection (1)(a)(ii)(B)(VI) that is greater than the result described in Subsection (1)(a)(ii)(A)(VIII) of 95% of the ensemble districts; or
(1)(a)(iii)(B) a result described in Subsection (1)(a)(ii)(B)(VI) that is greater than the result described in Subsection (1)(a)(ii)(A)(VIII) of 95% of the ensemble districts after culling the ensemble to include only redistricting plans that pass the partisan bias test, as described in Subsection (1)(c)(ii).
(1)(b) "Mean-median difference test" means an evaluation of a proposed redistricting plan:
(1)(b)(i) that calculates the difference between a party's average statewide vote share and the party's median vote share across all districts in a proposed redistricting plan; and
(1)(b)(ii) for which a difference described in Subsection (1)(b)(i) that is greater than a 2% deviation from the mean fails the mean-median difference test.
(1)(c) "Measures of partisan symmetry" means, for a congressional redistricting plan:
(1)(c)(i) the partisan bias test; and
(1)(c)(ii) an ensemble analysis with subsequent culling to include only redistricting plans that pass the partisan bias test to ensure the plan is within the statistical bounds of passing plans.
(1)(d) "Partisan bias test" means an evaluation of partisan symmetry in a proposed redistricting plan:
(1)(d)(i) that follows the following sequential steps:
(1)(d)(i)(A) calculate each party's statewide vote share using the partisan index;
(1)(d)(i)(B) calculate the difference between each party's statewide vote share and 50%;
(1)(d)(i)(C) subtract the difference described in Subsection (1)(d)(i)(B) from each party's vote share in each district in the proposed redistricting plan based on the partisan index data for each census block within the district; and
(1)(d)(i)(D) based on the adjusted vote share described in Subsection (1)(d)(i)(C), calculate the difference between each party's expected seat share and 50% of the total seats in a hypothetical election, with the difference between the party's seat share in the hypothetical election and 50% of the total seats representing the degree of partisan bias; and
(1)(d)(ii) for which a result described in Subsection (1)(d)(i) other than the following fails the partisan bias test:
(1)(d)(ii)(A) for an even number of seats, 0; or
(1)(d)(ii)(B) for an odd number of seats, 0.5.
(1)(e) "Partisan index" means an average of the partisan vote share:
(1)(e)(i) except as provided in Subsection (1)(e)(ii), in the three immediately preceding statewide elections for each of the following offices:
(1)(e)(i)(A) United States president;
(1)(e)(i)(B) governor;
(1)(e)(i)(C) attorney general;
(1)(e)(i)(D) state treasurer; and
(1)(e)(i)(E) state auditor;
(1)(e)(ii) except as provided in Subsection (1)(e)(iii), that excludes an election for an office in which the two largest political parties did not field a candidate; and
(1)(e)(iii) that allocates votes for an independent general election candidate to a political party if the independent candidate attempted to win the nomination of the political party at a primary election or party convention for the same general election.
(1)(f) "Sequential Monte Carlo simulation" means a probabilistic algorithm that simultaneously generates a representative ensemble of districting plans for comparison in an ensemble analysis by building redistricting plans through a step-by-step random sampling method, weighting and resampling the plans to reflect legal and geometric criteria.
(1)(g) "Unduly favor or disfavor" in regards to Subsection (4) for purposes of a congressional map, means the map is asymmetrical under the measures of partisan symmetry and fails the mean-median difference test.
(2) This section establishes redistricting standards and requirements applicable to the Legislature and to the Utah Independent Redistricting Commission.
(3) The Legislature and the Commission shall abide by the following redistricting standards to the greatest extent practicable and in the following order of priority:
(3)(a) adhering to the Constitution of the United States and federal laws, such as the Voting Rights Act, 52 U.S.C. Secs. 10101 through 10702, including, to the extent required, achieving equal population among districts using the most recent national decennial enumeration made by the authority of the United States;
(3)(b) minimizing the division of municipalities and counties across multiple districts, giving first priority to minimizing the division of municipalities and second priority to minimizing the division of counties;
(3)(c) creating districts that are geographically compact;
(3)(d) creating districts that are contiguous and that allow for the ease of transportation throughout the district;
(3)(e) preserving traditional neighborhoods and local communities of interest;
(3)(f) following natural and geographic features, boundaries, and barriers; and
(3)(g) maximizing boundary agreement among different types of districts.
(4) (4)(a) The Legislature and the Commission may not divide districts in a manner that purposefully or unduly favors or disfavors any incumbent elected official, candidate or prospective candidate for elective office, or any political party.
(4)(b) Absent clear and convincing evidence of purpose, a redistricting plan that is within the acceptable bounds of the ensemble analysis does not purposefully favor or disfavor a political party under Subsection (4)(a).
(4)(c) A redistricting plan that is symmetrical under the measures of partisan symmetry and passes the mean-median difference test does not unduly favor or disfavor a political party under Subsection (4)(a).
(5) The Legislature and the Commission shall use judicial standards and the best available data and scientific and statistical methods, including measures of partisan symmetry, to assess whether a proposed redistricting plan abides by and conforms to the redistricting standards contained in this section, including the restrictions contained in Subsection (4).
(6) Partisan political data and information, such as partisan election results, voting records, political party affiliation information, and residential addresses of incumbent elected officials and candidates or prospective candidates for elective office, may not be considered by the Legislature or by the Commission, except as permitted under Subsection (5).
(7) The Legislature and the Commission shall make computer software and information and data concerning proposed redistricting plans reasonably available to the public so that the public has a meaningful opportunity to review redistricting plans and to conduct the assessments described in Subsection (5).
(8) Any judicial review of a congressional redistricting plan to determine whether the Legislature or Commission complies with this section regarding purposefully or unduly favoring or disfavoring a political party shall base the review on the outcomes of the following, in accordance with this section:
(8)(a) an ensemble analysis;
(8)(b) the partisan bias test; and
(8)(c) the mean-median difference test.

Legislative History

Amended by Chapter 1, 2025 Special Session 1