A variety of discussion and resources that I’ve found related to No MTBF.
While researching reliability predictions I ran across this paper that critically examines a few common reliability prediction methods.
Jais, C., Werner, B. & Das, D., 2013, Reliability and Maintainability Symposium (RAMS), 2013 Proceedings-Annual, Reliability predictions-continued reliance on a misleading approach. pp. 1-6.
A draft version of the paper that was eventually published as “J.A.Jones & J.A.Hayes, ”A comparison of electronic-reliability prediction models”, IEEE Transactions on reliability, June 1999, Volume 48, Number 2, pp 127-134” A nice comparison and illustration of the variability to expect when using a parts count prediction method. Graciously provided by Jeff – and it’s in draft form, so please pardon the odd typo’s etc.
Bowles, J. B. and J. G. Dobbins (2004). “Approximate Reliability and Availability Models for High Availability and Fault-tolerant Systems with Repair.” Quality and Reliability Engineering International 20(7): 679-697.
Systems designed for high availability and fault tolerance are often configured as a series combination of redundant subsystems. When a unit of a subsystem fails, the system remains operational while the failed unit is repaired; however, if too many units in a subsystem fail concurrently, the system fails.
Under conditions usually met in practical situations, we show that the reliability and availability of such systems can be accurately modeled by representing each redundant subsystem with a constant, lsquoeffectiversquo failure rate equal to the inverse of the subsystem mean-time-to-failure (MTTF). The approximation model is surprisingly accurate, with an error on the order of the square of the ratio mean-time-to-repair to mean-time-to-failure (MTTR/MTTF), and it has wide applicability for commercial, high-availability and fault-tolerant computer systems.
The effective subsystem failure rates can be used to: (1) evaluate the system and subsystem reliability and availability; (2) estimate the system MTTF; and (3) provide a basis for the iterative analysis of large complex systems. Some observations from renewal theory suggest that the approximate models can be used even when the unit failure rates are not constant and when the redundant units are not homogeneous. Copyright © 2004 John Wiley & Sons, Ltd. Bowles, J. B. (2002).
“Commentary-caution: constant failure-rate models may be hazardous to your design.” Reliability, IEEE Transactions on 51(3): 375-377. This commentary examines the consequences, from a system perspective, of modeling system components using only their failure-rates, viz, the inverse of their MTTF, when, in actuality, they have a lifetime distribution with either an increasing or a decreasing hazard function.
Such models are often used because: they are tractable; they are thought to be “robust;” they depend only on average values; or only small amounts of data are available for calculations. However, the results of such models can greatly understate the system-reliability for some time periods and overstate it for others. Using MTTF as a figure of merit for system-reliability can be especially misleading. Furthermore, the redundancy in a redundant system might provide very little of the reliability improvement predicted by the constant failure-rate model, and series systems might, in fact, be much more reliable than predicted. The overall result is that a constant failure-rate model can give very misleading guidance for system-design
If you have a paper that you’d like to add to this list, please let me know.
Fred: One example I use when lecturing is the life time of a wine glass (or ceramic coffee mug). The probability of failure in the next day is independent of the age. Failure will occur when there is an accident (dropped on the floor or something in the dishwasher hitting it). Bill (email from William Q. Meeker Jr., Oct 26th, 2011)
Links on the Topic
Wikipedia does not have a great page, yet gets the idea across A Google search on 4/13/09 found over 2 million hits. The following are the best (imho) discussing MTBF, MTTF and their proper use. Weibull.com – a really good resource for the statistics treatment of many distribution useful in reliability engineering. This page treats MTTF and why it is not a good reliability metric.
FAQ extraction – what is MTBF RAC Journal 2nd Quarter 2005 lead article is titled Practical Considerations in Calculating Reliability of Fielded Products. An excellent short treatise on how to avoid the ‘issues’ with MTBF.
If you know of other great sites on the topic of MTBF – please let me know. firstname.lastname@example.org