Altered auditory feedback, so that people who stutter hear their voice differently, has been used for over 50 years in the treatment of stuttering.[64] Altered auditory feedback effect can be produced by speaking in chorus with another person, by blocking out the person who stutters' voice while talking (masking), by delaying slightly the voice of the person who stutters (delayed auditory feedback) or by altering the frequency of the feedback (frequency altered feedback). Studies of these techniques have had mixed results, with some people who stutter showing substantial reductions in stuttering, while others improved only slightly or not at all.[64] In a 2006 review of the efficacy of stuttering treatments, none of the studies on altered auditory feedback met the criteria for experimental quality, such as the presence of control groups.[65]