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In the measurement of hearing, a masking dilemma is a situation in which it is not possible to use masking to determine if the signal is crossing the head and activating the opposite cochlea. A nonmasking procedure, called the earplug procedure, was developed to rule out crossover for air-conducted signals in patients with a masking dilemma. In the procedure, an earplug is inserted in the suspect ear in an attempt to increase the loss in that ear. If the loss is increased significantly, then thresholds were obtained from the test ear and crossover is ruled out. If there was no significant threshold shift with the earplug, crossover is not ruled out and the uncertainty remains. Threshold shift criteria were developed for five test frequencies and for single test frequencies. The procedure correctly rules out crossover for a majority of unmaskable ears.