I've been on a Dick Ritger course many years ago now, and I think what you get from it will differ depending on the way you bowl when you go in.
If you don't have good fundamentals, like those taught by Ritger, then what he teaches will be a big departure from what you're used to doing, and so you will likely see your average drop after. When I attended the course, I had already built my game based on the kind of drills that Ritger espouses, and as a result saw my average increase by 20 pins or so for 6-8 months after.
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Reporting from England