Correspondents often ask whether there is research that establishes that my curricular materials are effective. The question is usually about the Lab Gear, but recently someone asked it about my pentomino materials.
Unfortunately, I don't know the answer. If someone did this kind of research, they didn't tell me about it.
What I do know is that many teachers over the years have told me how well my stuff works with their students, and how much they love it. Of course, that doesn't say much, since the people who hate it don't tell me!