Tutorial: How to Evaluate Your Map

Daniel McGlone
Sep 7 · 5 min read

When you create a plan in DistrictBuilder, we give you the tools to evaluate your districts with the metrics used to evaluate a plan as legal, valid, and fair. The National Conference of State Legislatures has a guide to commonly accepted redistricting criteria, or what is known as traditional districting principles. We have built-in support for most of the criteria and continue to expand on this work with new features in the pipeline.

To get started evaluating your district plan, click on the Evaluate button at the top right of your map.

When you open the Evaluate panel, you’ll see the Required evaluation metrics of Equal Population and Contiguity as well as Optional criteria. We have defined required metrics as those that a map must pass (the metric will have a green check) to be considered a valid redistricting plan.

Equal population

For Congressional districts, plans are usually expected to be within 1 person of the target population. For legislative districts, plans generally have a 5% deviation above and below the target, or 10% total deviation. This is built into DistrictBuilder anytime you create a map with a template of an existing chamber. However, when you create a plan, you can also adjust that to be a custom threshold. To check for equal population, click the button in the Evaluate panel to advance to the Equal Population tab.

On this tab, you can see a list of all your districts, whether they meet the deviation threshold, and the percentage and raw number deviation above or below the target. A map will also display so you can view the deviation by district.

Contiguity

Another common redistricting requirement is that all districts are contiguous. That means all parts of the district must be accessible to each other in a single, unbroken shape. Clicking on the Contiguity button will display a panel to check your districts.

If your entire plan is contiguous, you’ll see the green check to Contiguity.

Note that for some plans it may be impossible to generate contiguity due to islands or stranded geography. That’s usually permitted, but you should check to make sure that it can’t physically be connected with the rest of the district.

Competitiveness

Partisan gerrymandering has been a hot topic over the past decade with some courts believing that it should be outlawed when drawing a district plan. In general, it can be considered good practice to ensure that your district plan has a similar partisan lean as the state as a whole. The competitiveness metric displays the partisan voting index (PVI) of each district, which you can compare to the state average. For most states, this is using 2016 and 2020 data.

In addition, through our partnership with PlanScore, you can send your map to PlanScore for additional political evaluation metrics.

Compactness

Compact districts tend to resemble shapes like a square or circle. Our compactness metric is the Polsby-Popper score, which is a ratio of area to perimeter of the district. Higher percentages are better, however, this metric is very sensitive to the impacts of physical geography and shouldn’t necessarily be considered the only indicator of gerrymandering. Nevertheless, some states, such as Arizona and Florida, consider this a valid metric for evaluating districts.

With the compactness evaluation metric, you can see the compactness “score” for each district in your plan.

Majority-minority

The creation of majority-minority districts is required by law for most places where there is a substantial enough minority population. In addition, Section 2 of the Voting Rights Act of 1965 prohibits plans that intentionally or inadvertently discriminate on the basis of race, which could dilute the minority vote. Majority-minority districts can be created to remedy that section of the VRA, with the creation of districts that contain a majority of minority population enabling them to elect candidates of their choice. The Majority-minority evaluation metric lists which districts have a 50% +1 of a minority population. It will also display whether a district is a “Minority Coalition”, which is a district where the combined racial minorities make up a majority of the population.

County splits

The county split panel evaluates whether a county is split between two or more districts. Each county is listed on the left, indicating whether it is split. Generally, plans should respect county boundaries. However, that isn’t always possible when a county’s population is larger than the target district population.

We developed DistrictBuilder to make redistricting accessible, easy, and fair. We also want to give you the tools to evaluate your districts and plans the same way the professionals do. To that end, we continue to build additional features or metrics to support plan evaluation. Let us know what features you think would make great additions to DistrictBuilder.

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