The constitutional amendment approved by Missouri voters in November keeps those criteria. But it also requires a new nonpartisan state demographer to base state House and Senate districts on the votes cast in the previous three elections for president, governor and U.S. senator — races that are decided by voters statewide and are not affected by gerrymandering.
The districts must come as close as practical to achieving “partisan fairness” as measured by a formula called “the efficiency gap.”
That formula was created earlier this decade by Eric McGhee, a researcher at the nonpartisan Public Policy Institute of California, and University of Chicago law professor Nick Stephanopoulos. It compares the statewide share of the vote a party receives with the statewide percentage of seats it wins, taking into account a common political expectation: For each 1 percentage point gain in its statewide vote share, a party normally increases its seat share by 2 percentage points.
Read more at Associated Press