Reliability Measurement: MTBF vs MTTR (Answering the MCQ)

In reliability engineering, a common time-based metric for reliability is the MTBF (often used as an indirect reliability indicator). In contrast, MTTR measures maintainability/repair effectiveness rather than reliability.

For the given multiple-choice question (“Measure of reliability is given by”), the correct option among:

• (a) Mean time between success
• (b) MTBF
• (c) Mean reliable
• (d) MTTR

is (b) MTBF.

Mermaid overview of the relationship between these metrics:

Note: “Mean time between success” is not a standard reliability term; and “Mean reliable” is not a recognized metric name. MTTR is a repair/downtime metric and is typically categorized under maintainability.

Key reliability metrics and what they mean

1) MTBF

• MTBF estimates how long a system is expected to operate before failing again.
• It is widely used as a practical reliability indicator, especially in early design trade-offs and field reliability reporting.

2) MTTR

• MTTR represents the average time required to repair/restore a system after a failure.
• It affects downtime and therefore operational readiness, but it is not the same as reliability.

3) Availability link (why MTBF and MTTR appear together)

Although the question asks specifically for “measure of reliability,” it’s helpful to know why MTBF and MTTR are both used together in availability calculations.

A common form for steady-state availability is:

Where:

• Higher MTBF increases availability (more time operating)
• Lower MTTR increases availability (less downtime)

Keyword cues for this section:

• Reliability
• Availability
• Maintainability
• Failure rate