Visual tool measures stability of ranked choice election results
STV Audit Graphs: A Visual Tool to Measure Election Stability
Computer Science and Game Theory
Summary
Elections using the Single Transferable Vote (STV) system are complex because early decisions can affect the whole outcome. The authors present a new visual tool called audit graphs that helps check how stable or uncertain the election results are. This tool looks at nearby election scenarios by slightly changing some votes to see if the outcome might have changed. This approach helps to better understand and verify the accuracy of STV election results.
What this means in practice
- •For election officials: Use audit graphs to verify and quantify uncertainty in complex ranked-choice elections.
- •For voting software developers: Incorporate audit graphs into election systems to enhance auditability and election outcome verification.
Authors
Edouard Heitzmann
Abstract
The Single Transferable Vote (STV) is an algorithmic election rule. Round by round, a profile of ranked-choice ballots is reinterpreted to determine which decision to make next, and candidates are seated or eliminated until a full winner set emerges. This algorithmic nature makes STV theoretically more brittle than other election rules: uncertainty about an early round of the election might percolate irreversibly into the rest. For this reason, a generalized non-trivial Risk-Limiting Audit (RLA) framework has remained elusive for STV. Such a framework must concoct a set of null hypotheses, or assertions, whose rejection would bound the probability that the outcome of the election was incorrectly reported. We present audit graphs as a solution to design the assertions needed for RLAs of arbitrary STV elections, as well as quantitatively describe the uncertainty (or lack thereof) of their outcomes. These audit graphs explore election paths that are ``close'' to the recorded one by considering the alternative decisions the STV algorithm might have made if a small number of ballots were perturbed.