Coupled with probability is the duration over which the probability applies.
For example, if we desire 99 of 100 to survive, we must state over which period of time this applies. It is proper to state the couple of 99% reliable over 1 year.
It is not sufficient to define reliability as ‘5-year product’ as it does not contain the information related to how many are expected to survive the 5 years. Likewise, it is not sufficient to say a product has 5 – 9’s reliability (meaning the probability of failure is less than 0.00001) as it does not contain the duration.
If the product has high reliability for only a few seconds, that does not help us make judgements about the first year of life.
MTBF or MTTF are not durations. They are the inverse of the failure rate, thus a probability.
Do not use MTBF or similar metrics when working in reliability engineering.
Types of durations
A product once created has a finite lifetime. We use probability distributions to describe the product’s expected life over time. Time here implies calendar time. You may use distance traveled, operating hours, or cycles, depending on the product and dominant failure mechanisms.
Another group of durations is various points in time of interest to the business. Since early failures tend to impact customer satisfaction adversely, we may track failures that occur within the first month of use with a specific duration / probability couplet. For example, we may set a goal and monitor field performance for early life failures with a goal of 99.9% reliability over the 1st month of operation.
Many product include a warranty or some financial obligation to replace or repair a faulty product. The actual duration may vary by industry and markets. You organization may track this duration as if not predicted and managed accurately will impact the organization’s financial reporting.
The longer term reliability of a product impact customer loyalty and brand promise. Also, many design teams use a design life (period of time over which the product is expected to placed in service and provide value to the customer). The design life is often much longer than the warranty period and is an estimate of the useful life of the product.