**5.3 No-reference metrics**

When the reference is not available to design the objective quality method, for example in environments such as internet or video mobile, then it is necessary to utilize no-reference metrics to evaluate the degradation.

Most of the times, these kind of studies are focus on analyzing impairments due to artifacts, that degrade the perception of the user. So, the no-reference metrics are distributed in groups, depending on the artifact characterized.
