“What’s the MTBF of a Human?” That’s a bit of a strange question? I ask this question in my Reliability 101 course. Why ask such a weird question? I’ll tell you why. Because MTBF is the worst, most confusing, crappy metric used in the reliability discipline. Ok maybe that is a smidge harsh, it does have good intentions. But the amount of damage that has been done by the misunderstanding it has caused is horrendous. MTBF stands for “Mean Time Between Failure.” It is the inverse of failure rate. An MTBF of 100,000 hrs/failure is a failure rate of 1/100,000 fails/hr = .00001 fails/hr. Those are numbers, what does that look like in operation? Does it mean… The product lasts 100,000 hrs before failing? Half the population fails by 100,000 hours? Wait a minute! our product is only supposed to last three years with a 50% duty cycle. That’s 13,140 hrs of use. Why would we have an MTBF goal of 100,000 hours? It can’t even run that long if everything goes perfectly. Because of all this confusion what occurs is everyone makes up their own definition of what MTBF is. It’s usually one of the first two I listed, how long it runs or when half have failed. No one wants to sound stupid and ask what it means, so we all just pretend. This is the moral of “The Emperor’s New Clothes. ” We’ll I’m here to tell you that that dude is buck naked. That is why I ask the “MTBF of a Human” question. Because it forces a harsh realization when I give the correct answer. The answer is “At least 800 years.” The beauty is that the shared confusion in the room brings a sigh of relief as everyone realizes they weren’t alone in not knowing. “Dude go put some clothes on, you’re kinda freaking us out, and there are kids here!” Ok so I gave the answer. The MTBF of a human is 800 years. That’s actually very conservative. In your current lifestyle it is probably more like 2,000 years. A 800 year MTBF is more indicative of living in some very harsh old world conditions. Maybe a coal mining town in the 1700’s. The group’s surprise that a human can have an 800 year MTBF brings about a new interest in hearing some MTBF 101. Ok here we go….. When MTBF is used as a metric to describe a product’s reliability during it’s use life there are three assumptions.
- The first is that no “Infant Mortality” (i.e. quality failures) are included in this metric.
- The second is that no wear-out ( i.e. end of life) failures are included in this metric.
- The third is failures during use life occur randomly. So in any given moment during use life a failure is just as likely to occur as at any other moment. For a product with a ten year life this means that a random failure is just as likely to occur at three months of age as it is at seven years of age.
- We do not include children who die (<13 years of age). These are infant mortality. In the production world we consider these to be quality defect and not a characteristic of the design’s reliability.
- We don’t include retirees. In production these are items that are to be removed from service (retired). The manufacturer has predicted that wear out failure modes are going to become dominant at some point and that the promised use reliability will no longer be up held.
- We are not repairing systems
- Units that fail are being immediately replaced with new units that are past the infant mortality stage so the population is a consistent number.