Presenting a stimulus to observers and asking them to report whether or not they perceive it is the basic procedure for measuring thresholds. Biological systems are not fixed, however, but rather are variable in their reaction. Therefore, when an observer is presented on several occasions with the same stimulus, he or she is likely to respond yes on some trials and no on other trials. Thus, the threshold cannot be defined as the stimulus value below which detection never occurs and above which detection always occurs. The concept of the threshold has obviously been, and still is, useful, since it affords a technique for quantifying the sensitivity of sensory systems. But since reactions to stimuli are variable, the threshold must be specified as a statistical value. Typically, the threshold has been defined as the stimulus value which is perceptible in 50% of the trials.