Like most such studies, this begs more questions than it asks, and the answers must always be ‘it depends on how you do it’, so it is more than a bit odd that the question even arises. Of course some OERs are at least as good as some for-profit textbooks under some circumstances, and the converse is almost certainly true too. I have never heard a less-than-stupid argument that OERs are necessarily worse than paid-for resources, nor vice versa. It’s a ridiculous idea. The point about OERs is not that they are better or worse as educational resources per se but that they are open. This does make them much cheaper, which is no bad thing. The big advantage, though, is that they can be adapted more easily and freely for different contexts, without constraint. In principle, as a result, they can evolve to become better: though not all are used that way and only a few will improve in the process, that’s ultimately the most compelling advantage.
It is, though, good to see that OERs, as used at the moment, are at least as good as closed educational resources across a wide range of subject areas, and are sometimes better. I guess there might be someone somewhere who believes otherwise. If so, we can now give them a bit of empirical proof that they are wrong.