MTBF, Really?

Abstract

Chris and Fred discuss the MTBF … again. And again. People don’t (want to) get it. So here we go again …

Key Points

Join Chris and Fred as they discuss the MTBF and why it should virtually never be used. Why?

Topics include:

• What’s wrong with the MTBF when it comes to reliability? When we assume that the only thing we need to understand is the MTBF, we can never use reliability models that include any form of early wear-in or late wear-out. So, it means we assume a constant hazard rate, which means your thing never stays young and never gets old. That’s right, a 100-year-old product that is somehow still working is just as likely to survive the next day as one that comes out of the box.
• But when I assume (just) the MTBF, I get better results than when we do more detailed analysis. A Toyota Corolla has a 1.6 Litre engine. So does a F1 race car. Now let’s say that you measured the top speeds of both cars. For the F1 race car, we get 372.499 km/h or 231.46 mph. For the Toyota Corolla, we get 188.3 km/h or 117.0 mph. But let’s now say that we don’t like the top speed of the Toyota Corolla, and would like it to be higher. What you could do is pretend you didn’t measure the top speed of the Toyota Corolla, and then assume that because it’s engine is the same size as the F1 race car’s engine … we assume it has the same top speed as the F1 race car. Crazy right? … just as crazy as assuming an MTBF or constant hazard rate because you like the number you get better.
• Ostriches don’t actually put their heads in the sand … but many ‘reliability engineers’ do. When we ask some organizations and reliability engineers why they still use nothing but the MTBF, they say things like ‘we’ve never seen it be anything else.‘ And when we ask what, if anything, they have done to look for evidence to the contrary … ‘we just assume we are in the bottom of the bathtub curve.’ Some people don’t know that no system actually has a ‘bathtub curve’ that we see beautifully traced out in a textbook. So why are we still here?

Enjoy an episode of Speaking of Reliability. Where you can join friends as they discuss reliability topics. Join us as we discuss topics ranging from design for reliability techniques to field data analysis approaches.

SOR 965 MTBF, Really?Christopher Jackson

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