Page "Psychophysics" Paragraph 19

In experiments, the ascending and descending methods are used alternately and the thresholds are averaged.

A possible disadvantage of these methods is that the subject may become accustomed to reporting that they perceive a stimulus and may continue reporting the same way even beyond the threshold ( the error of habituation ).

Conversely, the subject may also anticipate that the stimulus is about to become detectable or undetectable and may make a premature judgment ( the error of anticipation ).