The Mean Opinion Score (MOS), as estimated using the E-model. By using the E-model to estimate MOS, provides you with a highly accurate prediction of how a current user would rate the voice quality of your VoIP network. Examining the Burst Percentage and Burst Density metrics can often give you a clue as to what is driving MOS scores down.
Note that the traditional method of obtaining a Mean Opinion Score is to survey actual users listening to sample sentences, rating audio quality from 1 (poor) to 5 (excellent). The E-model was developed because this type of testing is simply not practical for most VoIP installations, and in any case cannot be updated in real time.