This article is a follow-up to a 2016 study by the same author and updates the work with the last three years of research into the efficacy of and perceptions about open educational resources (OER). The article considers whether and how often the studies controlled for student and teacher variables, as well as the strength of the research methodology in general. I would think these types of studies would be more appropriate to open pedagogy rather than OER. And as always I'm not really comfortable with studies depicting OER (or education in general) as some sort of 'treatment', as though it were analogous to medicine. Maybe - maybe - you could do this study for a specific resource, but it's absurd to think you're getting useful data by studying 'efficacy' or 'perceptions' of OER in general.