EFFECT OF STEP-SIZE ON CLINICAL AND ADAPTIVE 2IFC PROCEDURES IN QUIETAND IN A NOISE BACKGROUND

Audibility thresholds for a 1000-Hz sinusoid were measured with a stan dard clinical (CLIN) procedure and a two-interval, forced-choice (2IFC ) adaptive procedure bracketing 79% correct. Both used 2- and 5-dB ste p sizes in quiet and in a continuous, broadband noise background. Clin ical thresholds were from 2 to 4 dB higher than 2IFC thresholds, depen ding on the condition. Step size had a larger effect on the CLIN thres holds than the 2IFC thresholds. For the CLIN procedure, thresholds wit h a 2-dB step size were 1.4 dB lower than with a 5-dB step size. For t he 2IFC procedure, thresholds with a 2-dB step size were 0.8 dB higher than with a 5-dB step size. Reliability, as measured by the intrasubj ect standard deviation, was better for the 2IFC than for the CLIN proc edure and better in noise than in quiet. Reliability was unaffected by step size. Adding extra trials to the 2IFC adaptive track decreased t he variability across threshold estimates, but more for the noise back ground than the quiet background. The efficiency of the 2IFC procedure was fairly constant across track length in noise, but decreased for l onger track lengths in quiet. In both quiet and noise backgrounds, CLI N procedures were much more efficient than 2IFC procedures.