Randi has run into retroactive excuses to explain failures to demonstrate paranormal skills so often that he has added another small demonstration to his tests. Prior to taking the test, Randi has subjects sign an affidavit stating that they agreed to the conditions of the test, that they would later offer no objections to the test, and that, in fact, they expected to do well under the stated conditions. At that point Randi hands them a sealed envelope. After the test, when they attempt to reject the outcome as poor experimental design, he asks them to open the envelope. The letter in the envelope simply states, “You have agreed that the conditions were optimum and that you would offer no excuses after the test. You have now offered those excuses.”

Douglas W. Hubbard, How to measure anything: Finding the value of “intangibles“ in business, Hoboken, 2014, p. 46