The short version of this post is "Check out David Bressoud's writing from the last year."
I am finally catching up on some reading blogs... David Bressoud in the October edition of Launchings has a useful and compelling overview of research evidence supporting IBL. I thought about writing my own version of this kind of post with the updated results from recent articles, but David Bressoud states things well, and send you to his blog.
Evidence for IBL has been increasing over time. One of the ways I like to phrase this is "all the vectors are pointing in the same direction." The nature of education research is that there will not be some gigantic airtight proof that method A is better than method B. It's the aggregate data that points us towards what is most likely to be true. Over time the strength of the signal has increased. What has been compelling is the aggregate evidence from across the levels of education and across disciplines.
On that note, I'd like to continue to encourage people to consider the "big tent" IBL framework. Engaging students and giving them opportunities to think, share, and do mathematics like mathematicians have value. There are many forms of IBL, including those that incorporate lectures. It's not a binary (on/off) choice. Hence, there are many entry points to active-learning pedagogies. Think Pair Share is an obvious place to start, and does not require wholesale changes in teaching practices. Find something. Find your IBL comfort zone!
Link: David Bressoud's Blog Post on Evidence for IBL