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.
The handbook · 7 volumes
Fora Soft Video Engineering Handbook
Absolute Category Rating is the quickest subjective test to run, but scores without a reference need careful panel and clip design. We wrote seven volumes covering the whole video stack, and for ACR start with Volume 1: subjective testing methods, MOS and ITU recommendations, and how human ratings calibrate PSNR, SSIM and VMAF. Volume 3 adds delivery and QoE, Volume 6 AI-based quality models. Written by Nikolay Sapunov, CEO at Fora Soft.
Planning an ACR subjective test and unsure how to design it?
Tell us what you want to compare: we design the ACR test with clips, panel size and scoring, then report MOS with confidence intervals. 250+ video products shipped since 2005.
