Jakob Nielsen (1994)
Nielsen Norman Group / Usability Inspection Methods.
URL: https://www.nngroup.com/articles/how-to-rate-the-severity-of-usability-problems/
Abstract. Defines the four-point severity scale (cosmetic / minor / major / catastrophe) that heuristic-evaluation reports have used ever since, plus the recommendation to have multiple raters score independently and take the mean. Operational complement to the original Nielsen heuristics paper.
Tags: heuristic-evaluation severity nielsen