The appropriateness of direct magnitude estimation and interval scaling procedures for assessing the speech intelligibility of hearing-impaired adults was investigated by determining whether the continuum of the talkers' intelligibility was prothetic or metathetic. The intelligibility of 20 hearing-impaired talkers was scaled by 20 listeners using direct magnitude estimation and by 20 listeners using interval scaling. The two sets of scaling data were related in the curvilinear fashion that is typical of prothetic continua, indicating better construct validity for direct magnitude estimation than for interval scaling of speech intelligibility.