I am absolutely astonished that the reliability profession and its noted experts are unable to develop a better metric to characterize reliability performance and specify reliability requirements. I respectfully submit that there is a simple and eloquent solution that has successfully been used in the ball bearing and machine industry for decades (that actually predates MTBF/MTTF), that should be considered as a replacement to MTBF/MTTF.
I am referring to the “Bx” Bearing Life metric which defines the life point in time (hours, days or years) or cycles when no more that x% of the units in a population will have failed. This reliability metric was developed in the ball and roller bearing industry, where a B10 life is a frequently used metric and requirement. B10 defines a life point (such as 10 years or 100,000 miles or 1 million cycles) when 10% of a population will fail by. In other words, the reliability is 90% at a specific point in life time that is appropriate to the type of equipment it is being applied to. Any value other that 10 can be used 5, 2, 1, 0.5 and even 0.1 are regularly used “Bx” failure risk values.