Feedback is self-evidently useful. Even without a teacher, we receive feedback on whatever we are trying to achieve and act accordingly. It all gets very messy when you try to define what good feedback is, and even messier when you try to measure it to prove the point! To me, feedback is simply part of good teaching and is almost impossible to isolate from it. It can even be silence at the appropriate time or even a certain look or a raised eyebrow. It's impossible to measure some things, and foolish to try. I've always found Socratic questions to work as well as (or to be honest, much better!) than any other feedback "technique" or style. I argued well over ten years to others in the profession that surely verbal feedback was more useful than written feedback is it is immediate, supportive, relevant and highly targeted. Yet, I was almost maligned for such things. As I like to say, it was like showing a dog a card trick. How times change. Additionally, it is never the burden of anyone to prove something doesn't work...!