7.4Defect Arrival Curves

What They Are

A defect arrival curve tracks the rate at which new bugs are discovered over time during a testing cycle. The shape of the curve tells you whether testing is nearing completion.

Curve Shapes and Their Meanings

Bugs Found Per Day

Case 1: Healthy (Converging)         Case 2: Unhealthy (Not Converging)

  │ ●                                  │     ●
  │   ●                                │ ●     ●
  │     ●                              │   ●     ●
  │       ●                            │       ●   ●
  │         ●  ●                       │             ●
  │              ● ●                   │
  │─────────────────→ time             │─────────────────→ time
  "Bug rate is decreasing.             "Bug rate is not decreasing.
   Testing is finding fewer issues.     We're still finding new areas
   Product is stabilizing."             with problems. Not ready."

How to Use Defect Arrival Curves

Curve Shape	Interpretation	Action
Steadily decreasing	Testing is effective, major issues found, product stabilizing	On track for release
Flat	Finding a consistent number of bugs per day	Testing is effective but the product has deeper issues; investigate root cause
Increasing	Each day finds more bugs than the last	Product quality is worse than expected; consider scope reduction or delay
Spike then decrease	A new area was tested or a new tester joined	Normal; the spike reflects expanded coverage
Near zero	Almost no bugs being found	Either quality is excellent or testing has exhausted its scenarios; try exploratory testing