On publication in 2009 John Hattie's Visible Learning presented the biggest ever collection of research into what actually work in schools to improve children's learning. Not what was fashionable, not what political and educational vested interests wanted to champion, but what actually produced the best results in terms of improving learning and educational outcomes. It became an instant bestseller and was described by the TES as revealing education's 'holy grail'.

Now in this latest book, Jo...