Mikael Laakso and Bo-Christer Björk have published "Hybrid Open Access—A Longitudinal Study" in the Journal of Informetrics.
Here's an excerpt:
This study estimates the development of hybrid open access (OA), i.e. articles published openly on the web within subscription-access journals. Included in the study are the five largest publishers of scholarly journals; Elsevier, Springer, Wiley-Blackwell, Taylor & Francis, and Sage. Since no central indexing or standardized metadata exists for identifying hybrid OA an explorative bottom-up methodological approach was developed. The individual search and filtering features of each publisher website and a-priori availability of data were leveraged to the extent possible. The results indicate a strong sustained growth in the volume of articles published as hybrid OA during 2007 (666 articles) to 2013 (13 994 articles). The share of hybrid articles was at 3.8% of total published articles for the period of 2011-2013 for journals with at least one identified hybrid OA article. Journals within the Scopus discipline categorization of Health and Life Sciences, in particular the field of Medicine, were found to be among the most frequent publishers of hybrid OA content. The study surfaces the many methodological challenges involved in obtaining metrics regarding hybrid OA, a growing business for journal publishers as science policy pressures for reduced access barriers to research publications.