The Mean Opinion Score (MOS) is Microsoft's quality measurement (0-5) of a user's experience in a Skype or Lync call. It assesses quality by measuring the network jitter, background noise, dropped packets, and other factors to score the user experience for a single device in a single call. Each device in a call has an inbound MOS and an outbound MOS.

The combined MOS score (and status) for a device is the LOWER value of the inbound MOS and outbound MOS scores in a call.