After Trials 14 and 15, and again after Trials 16 and 17, stimulus level is reduced by the new step size of 10 units, so that it is now below threshold at 120 units. On Trial 18, the participant reports that she cannot see the patch so, for Trial 19, it is incremented by 10 units, leading to detection on Trials 19 and 20, and a decrement for Trial 21. The oscillation about threshold continues until Trial 28, which does not take place because the change between Trials 27 and 28 is the 11th reversal, and so the end of the procedure. The computer then calculates the mean value at which the last 6 reversals occurred as follows: Reversal after Stimulus level Trial 20 130 Trial 21 120 Trial 23 130 Trial 24 120 Trial 26 130 Trial 27 120 ________________________ Mean 125 This is not a sophisticated procedure: step size is changed only once, and no attempt is made to calculate a psychometric function. Nevertheless, it illustrates several benefits of adaptive procedures. The experimenter can start with a high stimulus level, which it is almost certain that the participant will detect, and which shows the participant what a detectable stimulus should look like. The large initial step size rapidly brings stimulus level close to threshold, as indicated by the first few reversals, whereupon step size is greatly reduced, so that the size of
After Trials 14 and 15, and again after Trials 16 and 17, stimulus level is reduced by the new step size of 10 units, so that it is now below threshold at 120 units. On Trial 18, the participant reports that she cannot see the patch so, for Trial 19, it is incremented by 10 units, leading to detection on Trials 19 and 20, and a decrement for Trial 21. The oscillation about threshold continues until Trial 28, which does not take place because the change between Trials 27 and 28 is the 11th reversal, and so the end of the procedure. The computer then calculates the mean value at which the last 6 reversals occurred as follows: Reversal after Stimulus level Trial 20 130 Trial 21 120 Trial 23 130 Trial 24 120 Trial 26 130 Trial 27 120 ________________________ Mean 125 This is not a sophisticated procedure: step size is changed only once, and no attempt is made to calculate a psychometric function. Nevertheless, it illustrates several benefits of adaptive procedures. The experimenter can start with a high stimulus level, which it is almost certain that the participant will detect, and which shows the participant what a detectable stimulus should look like. The large initial step size rapidly brings stimulus level close to threshold, as indicated by the first few reversals, whereupon step size is greatly reduced, so that the size of