Dirk Pieper and Christoph Broschinski have published "OpenAPC: A Contribution to a Transparent and Reproducible Monitoring of Fee-Based Open Access Publishing Across Institutions and Nations" in Insights.
Here's an excerpt:
The OpenAPC initiative releases data sets on fees paid for open access (OA) journal articles by universities, funders and research institutions under an open database licence. OpenAPC is part of the INTACT project, which is funded by the German Research Foundation and located at Bielefeld University Library. This article provides insight into OpenAPC's technical and organizational background and shows how transparent and reproducible reporting on fee-based open access can be conducted across institutions and publishers to draw conclusions on the state of the OA transformation process. As part of the INTACT subproject, ESAC, the article also shows how OpenAPC workflows can be used to analyse offsetting deals, using the example of Springer Compact agreements.