Number of Subjects and their Selection

As stated in ITU recommendations, the number of subjects required to carry out the subjective quality test can vary from 4 to 40. Four is the absolute minimum for statistical reasons. The number of assessors needed depends upon the sensitivity and reliability of the test procedure adopted. The average number of subjects is about 15. They should not be directly involved either in picture or audio quality evaluation as part of their work and should not be experienced assessors. Prior to a session, subjects should usually be screened for normal visual acuity or corrected-to-normal acuity and for normal color vision (in the case of video quality evaluation).