The ITU-T standard, Measurements for Subjective Determination of Transmission Quality. Recommendation P.800 provides a standard subjective transmission quality evaluation process in the Conversation Opinion Test. The mean opinion score (MOS) is a mean average of the subjective evaluations of a group of trained volunteer listeners who rate voice transmission in terms of listening quality and listening effort on a scale of 1.0 (bad) to 5.0 (excellent). An MOS score of 4.0 (good) or better is considered toll quality. P.800 is an effective means of quantifying such a highly subjective perception, but it is expensive and time-consuming. In the mid-1990s, the ITU-T began the process of automating the objective measurement and testing of end-to-end voice quality across both circuit-switched and packet-switched networks. Those techniques include the following: