What is the prior probability of a proposed new treatment being superior to established treatments?

Editor–Terence Stephenson and David A Walker ask whether parents and children who are considering participating in a clinical trial of a new treatment for acute lymphoblastic leukaemia should be made aware that “for the past decade the new experimental treatments [studied in randomised trials] have given better outcomes.”1 In other words, hunches based on non-randomised evidence about which developments …