MTBF is commonly used as one of the indicators for understanding the frequency of failures.
MTBF (Mean Time Between Failures) refers to the average time between one failure and the next in products that are repaired and then returned to service(repair time is not included).

Failure rate： When the failure rate λ is constant, the reliability during the constant-failure-rate period follows an exponential distribution.

R(t) = e^-λt

In this case, MTBF is the reciprocal of the failure rate.

MTBF = 1 / λ

COSEL calculates MTBF (Mean Time Between Failures) using the following two methods:

1. Parts Count Method defined by EIAJ (now JEITA: Japan Electronics and Information Technology Industries Association)
This method is based on the JEITA standard RCR9102, which follows the parts count approach of MIL‑HDBK‑217D.
In this method, MTBF is calculated using the predetermined failure rate assigned to each component category.

2. Telcordia Parts Stress Analysis Method
This method is based on Telcordia SR‑332, a widely used reliability prediction standard among electronics manufacturers.
It estimates MTBF by calculating the failure rate of each component according to the electrical and thermal stresses (such as temperature and voltage) applied to it, and then summing the failure rates to determine the overall product MTBF.

※ Depending on the product, MTBF may be calculated using only one of the two methods above.