Reliability Engineering can be quite complex and it has, like other professions, its own language to avoid ambiguity. Words such as MTBF, MTTR, failure rate, availability, Weibull curves, chi-square demonstrations, etc. can confuse and mislead the average layperson.
I have come across people in my professional life who have mistakenly thought, through no fault of their own, that the term “MTBF” is the same as “service life” (or useful life). I’m not going to give a full definition of these terms as other sites such as http://www.weibull.com/hotwire/issue22/hottopics22.htm do a really good job.
However, just last week, I was attempting to explain MTBF (Mean Time Between Failure) and Useful Life to a colleague when I had the idea of using the famous “bathtub curve” to show that MTBF and Useful Life are not the same. I’m a visual guy, and I love using visuals to convey information in a manner that is simple (stripped to their essence) and not simplistic (stripped of their essence).
Let’s begin. Chart 1 indicates that a certain device has a Useful Life of 4.9 years (this would be typical of an enterprise-level hard drive). But notice that the MTBF is 1,000 years. That does not mean that these devices will last 1,000 years or even close to that figure. Ideally, each device should be replaced every 4-5 years depending on the acceptable failure rate (based on cost, spares availability, etc.).
Now look at Chart 2. The Useful Life of these devices is still 4.9 years, but the MTBF is now 100 years, not 1,000 years. So that should start getting you thinking; same Useful Life but different MTBF.
Finally we have Chart 3. The MTBF is 100 years (the same as the previous device) but the Useful Life is now 49.9 years and not 4.9 years. I think that should convince you that MTBF and Useful Life are not the same.
That’s it. I’m keeping this short and sweet. I hope that this provides a different and useful way of looking at MTBF and Useful Life, understanding that they are not the same, and understanding their relationship within the bathtub curve.
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