Mean opinion score (MOS)
Mean opinion score (MOS) is a rating of video or audio quality calculated by averaging scores given by human viewers, usually on a scale from 1 (bad) to 5 (excellent). MOS comes from controlled viewing tests where people rate sample clips. It remains the reference that automated metrics such as VMAF try to predict.
Why it matters
MOS is the ground truth for video quality, because it comes from real people. Codec choices, encoder settings and automated metrics are all judged against it. The catch is cost: panels take time and money, so companies use MOS to validate decisions rather than to monitor every video.
How it works
- A panel of viewers watches short test clips under controlled conditions, such as set lighting and screen distance.
- Each viewer rates each clip, often on a five-point scale from bad to excellent.
- The ratings for each clip are averaged to produce its mean opinion score.
- Results are checked for unreliable raters and reported with a margin of error.
What is a good MOS score?
On the common 1 to 5 scale, a MOS above 4 is generally considered good, and scores near 4.5 are close to excellent. Scores below 3 mean many viewers saw clear problems. Real panels rarely give a perfect 5, even for the original source, because some viewers never rate at the top of the scale.
MOS vs VMAF: what is the difference?
MOS comes from real viewers rating video, while VMAF is an automated score designed to predict how viewers would rate it. MOS is more trustworthy but slow and expensive to collect. VMAF is cheap and can run on every file. Teams use MOS tests to validate decisions, and VMAF to apply them at scale.
Where it's used
Broadcasters, OTT services and codec developers run MOS panel tests when choosing new codecs or encoder settings. Because tests are slow and costly, they run occasionally rather than on every video.
By the numbers
- The widely used absolute category rating (ACR) scale for MOS maps ratings from Bad to Excellent onto the numbers 1 to 5. (Wikipedia: Mean opinion score)
