Neutral redistricting criteria

Neutral criteria typically divide into two categories. One set of criteria are based solely on geography and demographic data. The other set depends on political data.

Population Equality

The most important piece of demographic data is the population. The goal is to have districts with nearly equal populations. For a state with k districts and a total population P, the quota for each district is P/k.

Ideally each district should have a population as close as possible to the total population. In order to achieve other geographic goals, it may be necessary to have small variations from the quota. One measure is the range, which is the difference between the largest and smallest district expressed as a percentage of the quota. State and federal laws limit the size of the range, and it can vary for different types of districts, for example state legislative districts may permit a larger range than congressional districts.

Another measure is the maximum deviation. This is the maximum absolute percentage difference permitted for a district from the quota. This is generally set by state or federal law, and may be set at half the maximum permitted range if range is the appropriate legal standard.

Contiguity

The most important geographic condition for districts is that they be contiguous. In its simplest form this means that all areas in the district touch the same area, that is there are no separate pieces that make up the district. The idea of dividing a large area into smaller contiguous pieces allows redistricting to be translated into a problem in graph theory.

Some states place restrictions on contiguity. The most common restriction is to bar point contiguity. point contiguity exists when an area of the district touches the rest of the district at only a single point. Two same-colored squares that are adjacent on a diagonal of a checkerboard are point contiguous.

States can also require that it be convenient to travel within a district. This requires knowledge of the roads in the state. For example there can be a requirement that all areas of the district be reachable from each other by road. Roads can be limited to public roads, all season roads, and can include ferries. Use of major highways to link larger areas of a district can also encourage districts that represent communities of interest.