Pair Comparison (PC), defined in ITU-T P.910 section 8.4 and called stimulus-comparison in ITU-R BT.500-15, shows the viewer two clips and asks the simplest possible question: which one is better? There is no rating scale, just a forced choice. Stripping away the scale removes the hardest part of any rating task - a viewer who cannot decide between a 3 and a 4 can almost always pick the one they prefer - which makes PC the most sensitive method when items are very close, the tool to separate encoders an absolute scale reports as tied. Turning the win-loss votes into a quality scale takes a model: Thurstone's Law of Comparative Judgment or the Bradley-Terry model, often in just-objectionable-difference units. The brutal weakness is arithmetic, not perception: distinct pairs grow as N times (N minus 1) over 2, so 12 conditions need 66 pairs and 40 need 780. It is reserved for small sets or run with active sampling (such as ASAP) that tests only the most informative pairs.
