Code Coverage
Coverage percentage is a proxy metric. Like most proxy metrics, it becomes counterproductive the moment teams start optimizing for it directly. The problem isn't coverage tools. The problem is treating a measurement of test quantity as a measurement of test quality. You can achieve 100% line coverage with
Testing
Unit tests verify specific behaviors on specific inputs. Property verification asks a different question: does this code satisfy a mathematical property for all inputs in a domain? The gap between these two approaches is the gap between "it worked on what I tested" and "it is correct.
Code Coverage
Coverage tools report four or five different metrics, and most teams pick one number to track without fully understanding what the others measure. That's a problem because each metric catches different categories of untested code, and conflating them gives you false confidence. This is the complete breakdown. Statement
QA
Defect density is a software quality metric that measures the number of bugs found per unit of code — typically per 1,000 lines of code (KLOC). It helps teams identify bug-prone modules, set quality benchmarks, and track improvement over time. Industry benchmarks range from 0.1 to 0.5