Any instrument designed to assist in the measurement of subjective evaluations of, or reactions to, a person, object, event, statement, or other item of interest.
Several varieties of rating scales have been developed. One common form of rating scale presents the rater with a spectrum of potential responses that includes antithetical elements at each end of a range of intermediate possibilities, on which the rater is expected to indicate the position that most accurately represents the rater's response to the subject in question. Another form of rating scale presents the rater with a list of characteristics or attributes from which the rater is expected to select those which the rater believes apply to the subject in question. Rating scale instruments are used in psychological research primarily to assess qualities for which no objective measurement techniques have been developed.
Bech, Per. Rating Scales for Psychopathology, Health Status, and Quality of Life. New York: Springer-Verlag, 1993.