An experiment was performed to find out how well human listeners could determine whether or not two different utterances were spoken by the same speaker. The speech was coded in three ways: high‐quality PCM (natural speech), linear prediction encoding (LPC), and ADPCM at 24 kbps. For this experiment all combinations of these three coding methods were used. A group of 16 reference speakers (customers—8 males, 8 females) along with 78 test speakers (16 reference and 62 imposters) were used in the experiment. Two test utterances were used: one for the male speakers and one for the female speakers. The 30 naive listeners who participated in the experiment were required to make same/different judgments. The results will be discussed in terms of the imposter acceptance error rates (miss rates) and customer rejection error rates (false‐alarm rates) for the different test conditions.