A Mean Opinion Score (MOS) is a numerical measure of the human-judged overall quality of an event or experience.  In telecommunications, a Mean Opinion Score is a ranking of the quality of voice and video sessions.

Most often judged on a scale of 1 (bad) to 5 (excellent), Mean Opinion Scores are the average of a number of other human-scored individual parameters.  Although originally Mean Opinion Scores were derived from surveys of expert observers, today a MOS is often produced by an Objective Measurement Method approximating a human ranking.


How is Mean Opinion Score Determined?

Mean Opinion Scores, as commonly used today, originated from polls of test subjects listening to audio or observing video. A number of current standards can be traced back to expert listeners and observers in distraction free quiet rooms subjectively logging experience scores. A MOS itself is an average from a number of individual components of session quality.

Nowadays, audio and video communications are not scored by a panel of individuals, but by a number of algorithms (Objective Measurement Methods) that attempt to approximate human experience. 

The most commonly used rating scale is the Absolute Category Ranking (ACR) scale, which ranges from 1 to 5. The levels of the Absolute Category Ranking are:

Excellent
Good
Fair
Poor
Bad

Due to the human tendency to avoid perfect ratings (now reflected in the objective approximations), somewhere around 4.0 - 4.5 is considered an excellent quality target. On the low end, call or video quality becomes unacceptable below a MOS of roughly 3.3.


What Causes a Low Mean Opinion Score?

All links in the chain from sender to receiver can cause a drop in mean opinion score. Everything from a human's health to audio and video equipment to computer settings can cause a degradation in communications quality. However, network effects are most readily apparent and measurable on these calls - jitter, latency, and packet loss lend themselves to numerical measurement, and have a direct effect on perceived call quality.