Frequency Discrimination as Measured by AB and ABX Procedures

It is often reported that difference thresholds for frequency when measured by an ABX procedure are at least twice as great as those measured by an AB procedure. The relationship has always been rationalized in terms of the greater phsyical and judgmental complexity of the ABX procedure. The present experiment was performed at 120 cps, 70 dB SPL, with two practiced subjects. For frequency differences of plus or minus 0.3 cps, or 0.25%, responses were 95% correct for ABX, and 82% correct for AB. This result contradicts the usual findings, and suggests a need for a new analysis of frequency-judgment mechanisms.