Click here to purchase the entire book in PDF format. Three-alternative forced choice (3AFC) There is an interesting alternative to the A/B/X test that is just as easy to set up and just as easy for the subjects to do, but that might give you better results. In the A/B/X test, the subject knows that A and B are different, and has to select whether X is the same as A or X is the same as B. Therefore, the subject has a 50% chance of getting the right answer. What if you presented the three stimuli to the subject and called them, say, A, B and C. Tell the subject that two of the three stimuli are identical and get them to choose the different one. Now, if the subject is guessing, he or she only has a 33.3% chance of getting the correct answer, so you can get the same results as an A/B/X test with the same degree of certainty in your responses, but with fewer questions.