Since ABX tests typically involve only a single listener performing multiple trials, the number of trials should be kept reasonably small to prevent listener fatigue from affecting the results. 16 trials is generally agreed to provide a good balance, and corresponds with Meilgaard's recommendation for multiple listeners in a duo-trio test, at least as far as minimizing type I errors is concerned. Typically, 0.01 is used as the critical type I error. So that would mean that a difference is concluded to be heard when 13 correct identifications out of 16 is achieved.