It's not proof as I don't know what they were like before.

I've seen/heard one person before he went on this course and a couple of times (2nd or 3rd year now, I think) since and in the session environment where I've seen him, it's not clear to me by listening what the course has done for him. He was a bloody good player before...

In his case (and I've not had the chance to ask him), the assessment I'd be most interested in is his own. Perhaps some might not regard a student's own assessment, but in terms of what he's gained, I feel it would be more reasonable and accurate than any I'm capable of making by listening.