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4.2Defect Metrics

Defect Density

What it measures: The number of defects relative to the size of the software.

Formula:

Defect Density = Number of Defects / Size of Software

Size can be measured as:
  - Lines of code (KLOC = thousands of lines of code)
  - Function points
  - Number of user stories
  - Number of modules

Example:

Release v3.2: 45 defects found in 120 KLOC
Defect Density = 45 / 120 = 0.375 defects per KLOC

Release v3.3: 32 defects found in 95 KLOC
Defect Density = 32 / 95 = 0.337 defects per KLOC

Trend: Improving (density decreased by 10%)

Why it matters: Defect density normalizes for project size. A project with 100 bugs in 500 KLOC is healthier than a project with 50 bugs in 10 KLOC.

Typical targets: 1-10 defects per KLOC for commercial software. Mission-critical systems target < 0.5 per KLOC.

Defect Escape Rate

What it measures: The percentage of defects that escape testing and reach production.

Formula:

Defect Escape Rate = (Defects Found in Production / Total Defects Found) x 100

Where Total Defects = Defects Found in Testing + Defects Found in Production

Example:

Sprint 47:
  Defects found in testing: 18
  Defects found in production: 2
  Total: 20
  Defect Escape Rate = (2 / 20) x 100 = 10%

Why it matters: This is the single most important QA effectiveness metric. It directly answers: "How good are we at catching bugs before customers see them?"

Typical targets: < 5% for mature teams. > 15% indicates significant testing gaps.

Defect Removal Efficiency (DRE)

What it measures: The percentage of defects removed before release.

Formula:

DRE = (Defects Found Before Release / Total Defects) x 100

Where Total Defects = Pre-release Defects + Post-release Defects

Example:

Release v3.2:
  Defects found before release: 45
  Defects found after release (within 90 days): 5
  DRE = (45 / 50) x 100 = 90%

Why it matters: DRE is the inverse perspective of escape rate. A DRE of 90% means you catch 9 out of 10 bugs before they reach users. Industry leaders aim for > 95%.

Relationship to escape rate:

DRE = 100% - Defect Escape Rate