Spanning Trees and Redistricting: New Methods for Sampling and Validation

Deciding whether a political districting plan was distorted by a hidden agenda, or whether it dilutes the voting power of some group, requires a neutral baseline for comparison. Remarkably, all nine U.S. Supreme Court justices have now signed on to decisions finding that computational methods can provide key evidence. Today, the leading approaches to the benchmarking of districting plans are based on the use of spanning trees for sampling graph partitions. We present a new reversible recombination algorithm and rigorously prove its fundamental properties. Furthermore, we argue for a canonical sampling distribution called the spanning tree distribution that is well adapted to redistricting and provides a principled foundation for comparing and validating methods. Together with a highly efficient (and open-source) implementation that can generate and handle large datasets, this work provides the most powerful null model to date for the gerrymandering problem, meeting an urgent democratic challenge with sound scientific methodology.