Of interest "Visible Learning: A synthesis of over 800 meta-analyses relating to achievement. ...a culmination of 15 years of research incorporating more than 50,000 studies and over 800 meta-analyses involving millions of students and represents the largest collection of evidence-based research into what actually works in schools to improve learning."

http://www.amazon.com/Visible-Learning-Synthesis-Meta-Analyses-Achievement/dp/0415476186 http://bookzz.org/book/969257/51d5f6