Source: https://www.amazon.com/Designing-Virtual-Worlds-Richard-Bartle/dp/0131018167/ref=as_li_ss_tl?ie=UTF8&qid=1468269314&sr=8-1&keywords=bartle+designing+virtual+worlds&linkCode=sl1&tag=atheoroffunfo-20&linkId=aacb200752a8c0a1ef11340132c50768