This plugin is deprecated since SonarQube 4.0. Please move to the concept of Technical Debt introduced in SonarQube 4.0.
The Quality Index plugin combines a global measure (quality index) of quality with a measure of the method complexity (complexity factor).
The Quality Index measures 4 weighted axes of quality and combines them to give a global score between 0 and 10 to the project, 10 being the best.
Coding issues: a PMD compliance index
Coding = (Blocker * 10 + Critical * 5 + Major * 3 + Minor + Info) / ValidLines
Complexity = (Complexity>30 * 10 + Complexity>20 * 5 + Complexity>10 * 3 + Complexity>1) / ValidLines
Coverage = Code coverage by unit tests.
If there are no unit tests and thus no code coverage, the Coverage axis will get the worst grade.
Style issues: a Checkstyle compliance index
Style = (Errors * 10 + Warnings) / ValidLines * 10
Quality Index = 10 - 4.5 * Coding - 2 * Complexity - 2 * Coverage - 1.5 * Style
The Complexity Factor measures the complexity density.
Complexity Factor = (5 * Complexity>30) * 100 / (Complexity>1 + Complexity>10 + Complexity>20 + Complexity>30)
Run a new quality analysis and the metrics will be fed.