Every 10 years, state legislatures, legislative commissions or separate bodies are tasked with drawing new boundary lines after the decennial census. Courts also occasionally intervene, but typically only in civil rights cases.
The Azavea analysis evaluates geographic compactness of each district using four different formulas. The Polsby-Popper ratio, generally the most widely cited metric, and Schwartzberg ratio measure the perimeter and indentation of a district. These measures essentially compare how closely an area resembles a circle to determine compactness. The two other metrics Azavea used – the Convex Hull and Reock ratios – assess how well dispersed districts are.
It's the less compact districts that are often deemed to be gerrymandered.
The firm noted, though, that a state's geography and coastal boundaries also affect compactness. Azavea tried to control for this, but acknowledged natural geographic boundaries do play a role in districts' compactness. In addition, states with only a few congressional districts also tend to have lower average compactness scores.
The table below shows Azavea's average district compactness for all states having more than one district, with lower ratios (multiplied by 100) representing the least compact districts:
