Absolute Category Rating (ACR) is the single-stimulus method defined in ITU-T P.910 section 8.1: each clip is shown once, on its own, and rated immediately afterward on five labelled categories - 5 Excellent, 4 Good, 3 Fair, 2 Poor, 1 Bad. Absolute means the viewer judges the clip on its own merits, with no reference to compare against, the way a real viewer meets a video in the wild. Average those ratings across the panel and you have the clip's MOS. ACR is the workhorse of the field because it is the fastest method per rating - one clip, one vote - which is why almost every large quality database and crowdsourced test is built on it. Its weakness is the flip side of that speed: with no reference visible, ACR may be insensitive to small impairments a double-stimulus method catches, and P.910 reports it rarely resolves a MOS difference below about 0.5 points with 24 subjects. The score is also contaminated by how much the viewer liked the content.
