This site is part a long string of attempts to eradicate the improper use of MTBF. This week two people have sent me references to work previously done and Chris sent me another podcast also highlighting issues with MTBF. Jim McLinn wrote about the possible transition away from constant failure rate back in 1990. Jim’s article titled “Constant failure rate – A paradigm in transition?” has the following abstract:
As a science, reliability has now entered middle age, having achieved almost 40 years of recognized modern practice. As we move into the new decade of the 1990s it is appropriate that we review the status of modern reliability. The history of science has lessons for us to learn concerning the nature of paradigm changes. Clearly some reliability practitioners have had difficulty changing their own world views as scientific knowledge has increased and the conditions of the profession have changed.
The very first generalized model for reliability was based upon electron tube life data from the early 1950s. It was with these old types of complex and failure prone products upon which the original reliability model was developed and generalized. This first model dictated that the failure probability density of electronics follows the exponential law which implies that the electronics will show constant failure rates during their useful lives. To get to the constant failure rate period, an infant mortality was traversed and about 10 per cent failures observed. The reliability beliefs of the 1950s when combined with the product successes of the 1960s has created a reliability paradigm problem that first became apparent in the late 1970s. Improved quality and design effort with new technologies and knowledge about effective screening changed the conditions. Modern semiconductor electronic products do not follow the original electron tube reliability model. The applicability of this original model and the subsequent thinking that it led to must now be questioned. It is time to create a new and better paradigm to replace the defunct exponential law.
McLinn, J. A. (1990), Constant failure rate—A paradigm in transition?. Qual. Reliab. Engng. Int., 6: 237–241. Then in 2001 Jeff Jones wrote the article titled “Estimation of System Reliability Using a “Non-Constant Failure Rate” Model”. The abstract follows:
One of the most controversial techniques in the field of reliability is reliability-prediction methods based on component constant-failure-rate data for the estimation of system failure rates. This paper investigates a new reliability-estimation method that does not depend upon constant failure rates. Many boards were selected from the Loughborough University field-reliability database, and their reliability was estimated using failure-intensity based methods and then compared with the actual failure intensity observed in the field. The predicted failure-intensity closely agrees with the observed value for the majority of a system operating lifetimes. The general failure intensity method lends itself very easily to system-reliability prediction. It appears to give an estimate of the system-reliability throughout the operating lifetime of the equipment and does not make assumptions, such as constant failure rate, which can be detrimental to the validity of the estimate. The predictions seem, on present evidence, to track the observed behavior well, given the uncertainties that are evident in the field. The failure intensity method should be investigated further to see if it is feasible to estimate the system reliability throughout its lifetime and hence provide a more realistic picture of the way in which electronic systems behave in the field.
Jeff Jones and joe Hayes,
Estimation of System Reliability Using a “Non-Constant Failure Rate” Model, IEEE TRANSACTIONS ON RELIABILITY, VOL. 50, NO. 3, SEPTEMBER 2001
And, more recently, Chris Peterson, in her daily blog, Test to be your Best, continues to alert us to the perils of MTBF and it’s associated mindset. So, why do we still need to remind ourselves and still need to educate others on how to properly use and when not to use MTBF? Apparently, the battle continues. If you’d like to help – let others know about this site and encourage the widespread eradication of MTBF. Let me now how it’s going.
Leave a Reply