Comparative MOS ratings allow a test sound to score above the reference.

Evidence

The comparative MOS is the third basic MOS scale, and it is typically defined to have values between −3 and 3. It can thus be used where given test case can be ranked better than reference. This can occur, for example, in speech enhancement techniques. — Ville Pulkki, Matti Karjalainen, Communication Acoustics- An…, 2350

Categorization

• DEFINE: Introduces the comparative MOS (CMOS) scale from -3 to 3 for ranking against a reference.