What this calculator does
It starts with observed cumulative MTBF, total test time divided by cumulative failures, then converts that average to the fitted instantaneous MTBF at the test endpoint.
When to use it
Use it to interpret a completed power-law reliability-growth record when β has been estimated from suitable ordered failure data. It does not determine whether a contractual reliability requirement has been demonstrated.
Inputs and formula
Test time and MTBF use the same unit. Failures are the positive integer events included under the declared test definition. The fitted slope must be at least zero and less than 1.
Cumulative MTBF = test time ÷ failures; end-of-test instantaneous MTBF = test time ÷ [failures × (1 − β)]Worked example and interpretation
1,500 test hours, 8 cumulative failures, and β of 0.437 give an observed cumulative MTBF of 187.5 hours and fitted end-of-test instantaneous MTBF of about 333.04 hours.
Practical notes and limitations
Do not use zero failures with this estimator. The point result has no confidence statement and is sensitive to the fitted slope. Report the failure definition, corrective-action policy, configuration history, test profile, data-fit method, and uncertainty beside it.