What is Defect Age?

Defect age is a measure of how long a defect has existed, either as elapsed time (from when it was introduced or detected to when it was fixed or closed) or as the number of phases between where it was introduced and where it was found.

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IN DEPTH

In depth.

Defect age comes in two related flavors. Time-based defect age measures the duration a defect stays open, for example, from detection to closure, useful for spotting bugs that linger unresolved. Phase-based defect age (sometimes called defect latency) measures how many development phases passed between when a defect was introduced (say, requirements) and when it was caught (say, production), highlighting how late escapes are being found.

The metric matters because cost grows with age. A requirements defect caught in requirements is cheap; the same defect caught in production is far more expensive to fix and may have caused real harm, the well-known economics behind shift-left testing. Tracking phase-based defect age shows whether the team is catching issues early or letting them slip through to expensive stages.

Time-based defect age helps manage the backlog: defects aging without resolution may indicate triage problems, unclear ownership, or under-prioritized risk. Like all metrics, defect age is a signal to investigate, not a target to game; aging defects and large introduce-to-detect gaps both point to process improvements (better early testing, faster triage).

WHY IT MATTERS

Why interviewers ask about this.

Defect age is a quality-metrics interview topic, especially for lead roles. Knowing both senses, time open and phases between introduction and detection, and connecting the latter to the rising cost of late defects and shift-left, shows you can use metrics to drive earlier detection and faster resolution.

EXAMPLE

Example scenario.

A team reviews defect age and finds several bugs that were introduced during design but only caught in production, a large phase-based age signaling weak early testing. They also see some defects open for months (high time-based age) due to unclear ownership. Both insights drive concrete fixes: earlier reviews and clearer triage.

TIP

Interview tip.

Define defect age as how long a defect has existed, by elapsed time (open duration) or by phases between introduction and detection (latency). Tie phase-based age to the rising cost of late defects and shift-left, and time-based age to backlog and triage health. Frame it as a signal to investigate, not a target.

FAQ

Frequently asked questions.

What are the two meanings of defect age?

Time-based defect age is the elapsed time a defect stays open (e.g., detection to closure), useful for spotting lingering bugs. Phase-based defect age (defect latency) is the number of development phases between where a defect was introduced and where it was found, highlighting how late escapes are being caught.

Why does defect age matter?

Because the cost of a defect rises the longer it goes undetected and unfixed, a requirements bug caught in production is far more expensive than one caught in requirements. Tracking defect age reveals whether the team catches issues early (shift-left) and resolves them promptly, pointing to process improvements.