Source: https://www.amazon.com/Visible-Learning-Teachers-Maximizing-Impact/dp/0415690153/ref=as_li_ss_tl?ie=UTF8&qid=1476462840&sr=8-1&keywords=visible+learning&linkCode=sl1&tag=learisgrow-20&linkId=6de0f4dadd186a39aa27e289c449eaf1