Source: https://www.amazon.com/Visible-Learning-Mathematics-Grades-K-12/dp/150636294X/ref=as_li_ss_tl?_encoding=UTF8&psc=1&refRID=4VYZ6HQM1SM73X5N2PCP&linkCode=sl1&tag=learisgrow-20&linkId=b80dd692234962792792cd07b1d99804