The Role of Anchors and Category Scale Labels in Speech Quality Rating

Abstract

Although there is widespread use of category scales for the evaluation of speech quality, the labeling and instructions to subjects about the use of such scales has frequently been intuitively selected. In this experiment, subjects rated speech stimuli varying in volume and circuit noise level on a verbal (“Unsatisfactory”-“Excellent”) or a numerical (1–5) scale. Subjects using the verbal scale were instructed either to define the endpoints of the scale as equal to the highest and lowest signal/noise ratios in the experiment (external anchors) or to use their telephony experience to define the scale (internal anchors). The subjects using the numerical scale used external anchors only. On the verbal scale, the internal anchoring resulted in a higher mean rating and more peaked response distribution than the external anchoring; this indicates that subjects can apply this scale somewhat independently of the stimulus distribution. In comparing the verbal and numerical scales, the response distributions show that there are small differences in category boundaries between the two scales. In general, the results suggest that anchoring plays a more important role than scale labeling in determining the subjects’ response distributions.