Subjective quality is video quality as real people perceive it, measured the only way perception can be measured directly: by asking humans. In a subjective test, viewers watch clips under controlled conditions, a calibrated display, a fixed viewing distance, and steady lighting, and rate each one, most often on the five-point Absolute Category Rating scale from 5 excellent to 1 bad. Averaging the ratings yields the Mean Opinion Score, the ground truth the whole field rests on. The catch is cost and care: a valid result needs at least fifteen observers and a reported confidence interval, because one person's opinion is noise and an uncontrolled screen or leading instructions produce confident numbers that mean nothing. Subjective quality is the truth that objective metrics are trained and validated to predict; when a properly run subjective test and a metric disagree, the subjective test wins and the metric has hit a blind spot.
