Mean opinion score

Mean opinion score, usually shortened to MOS, is a measure used in quality of experience work and telecommunications engineering to express the overall quality of a stimulus or system as a single number. It is the arithmetic mean of the values that test subjects assign on a predefined scale when they rate the performance of a system. Scores are normally gathered in a subjective quality evaluation test, although they can also be estimated by algorithms called objective quality models. The method is widely used for video, audio and audiovisual quality, and is not limited to those. The usual result is a rational number between 1 and 5, where 1 is the lowest perceived quality and 5 the highest, and the Absolute Category Rating scale maps verbal ratings from Bad to Excellent onto those numbers. The ITU-T defines several ways of naming a MOS in its recommendations, depending on whether the score came from listening, talking, conversational, video or audiovisual tests. Because categorical scales are ordinal and are affected by range-equalization bias, a MOS has meaning only relative to the test that produced it, and values from different tests should not be compared directly.