Many parameters of the viewing conditions can influence the results, such as room illumination, display type, brightness, contrast, resolution, viewing distance, and the age and educational level of experts.
There are an enormous number of ways of showing video sequences to experts and to record their opinions. A few of them have been standardized. They are thoroughly described in ITU-R recommendation BT.500. One of the standardized methods is DSIS - Double Stimulus Impairment Scale: the expert is presented with an unimpaired reference video, then with the same video impaired, and after that he is asked to vote on the second video using an impairment scale (from "impairments are imperceptible" to "impairments are very annoying").
Further refined testing methods to take into account low picture resolutions (VGA, CIF and QCIF), e.g. for mobile and multimedia applications are referred to in ITU-T Rec. P.910. Advanced setups for typical artefacts of high resolution (HDTV), e.g. in next generation networks incl. IPTV are also under development within the Video Quality Experts Group (VQEG).
Opinions of experts can be averaged; average mark is usually given with confidence interval. Additional procedures can be used for averaging, for example experts who give unstable results can be rejected (for instance, if their correlation with average opinion is small)...