Reduction of rating scale data by means of signal detection theory

A d’-index computed from ratings of psychological stimuli does not necessarily represent the unbiased psychological distance between these stimuli. Such ratings may not be considered as discrimination tasks in the signal detection sense, because the rater has to discriminate between the psychological impact of the stimuli on some internal continuum rather than between the stimuli as such. The rater therefore can use stimulus-specific criteria for his decisions. As a result, the d’-index computed from the rating data will be biased.