Hi Gavin
There are all sort of spurious stats out there that support whatever people want to talk about, but this is one I've come across a few times now and as it's university based is, hopefully, more rigorous than some others.
I haven't checked it out in huge detail but do go and have a look. It's from the National Training Laboratories in Maine and they have a pyramid of most and least effective training methods
lowery.tamu.edu/Teaming/Morgan1/sld023.htm
(sorry haven't worked out how to insert a link yet)
Also I found a couple of interesting academic sites that may be worth investigating further.
www.education.auckland.ac.nz/staff/j.hattie/
www.geoffpetty.com/
This is a really useful question to ask for all of us and will encourage me to keep a list of useful links from now on. THanks.