If you’ve done Usability Testing sessions you know that at the end of running a couple of sessions you endup with an endless list of comments, issues and notes. While every comment will be useful at some point, we need a way to filter through them and organise them into useful and actionable feedback.
The method that makes most sense to me is to rate the issues in terms of severity. And the scale I usually use was created by Dumas & Redish (1993), it only has four levels and they are very clearly defined: Cosmetic - enhancements and suggestions (e.g.: visual aspects); Minor - problems have a minor effect on usability (e.g.: might create temporary confusion); Serious - creates significant delay and frustration (e.g.: multiple tries necessary); Catastrophic - prevents task completion (e.g.: system is down).
This scale is specifically created for usability issues, if you are evaluating other aspects of the product such as Interface Design, Brand, Copy, Performance you might need to pick another scale or adapt an existing one.