Various summary measures of these outcomes, first half of 20th century.
I created this repository for the following blog post: "How often did 'proportional RCV' have 'a bad year'?"
If using the data, please cite:
Santucci, Jack. 2022. More Parties or No Parties: The Politics of Electoral Reform in America. Oxford University Press. https://doi.org/10.1093/oso/9780197630655.001.0001.