Despite standing for the time between failures, MTBF does not represent a duration. Despite having units of hours (months, cycles, etc.) is it not a duration related metric.
This little misunderstanding seems to cause major problems.
MTBF Calculation From Data
If I have 10 pieces of equipment and they have run for a year, 8,760 hours. And, during that year we enjoyed 5 failures, which were quickly repaired, what is the MTBF of that equipment?
10 units running for 8,760 hours for a total operating time of 87,600 hours. And 5 failures is the only other bit of information need for the calculation. 87,600 divided by 5 is 17,520 hours MTBF.
MTBF, Duration, and Confusion
Of the ten pieces of equipment that each operated for a year and experienced 5 failures, how does a mean time between failures of 17,520 hours remain consistent with the idea (mistaken idea) that we should only have 1 failure every 17,520 hours for each piece of equipment?
Well, it is consistent if you consider we are expecting 1 failure every 17,520 hours, and 17,520 divided by 8,760 hours is 2. Therefore we expect each piece of equipment to have a 50% chance of failure each year. 10 times 50% is 5, which what we experienced.
The confusion occurs when some expect all 10 units to run 2 years and only have one failure. Or that each unit should operate 17,520 hours then have a failure (this is less common to consider MTBF a failure-free period, yet it occurs).
MTBF is an Inverse Failure Rate
Keep in mind that we can consider MTBF as a probability of failure. Unit wise it is an inverse failure rate or the chance of failure per hour.
In the example above we have a 1 in 17,520 chance of failure every hour. Of course, ignoring early life and wear out patterns, which one should never do, btw. More more hours the equipment runs the more times we have a 1 in 17,520 chance of failure. Run for two years you are pretty much certain to have at least one failure.
MTBF does provide a chance per unit (in many cases an hour) of failure, it doesn’t mean the failure rate is accurate or fixed over any period of time we want to use.
In the example above we have data for one year of operation for the 10 units. We do not have information over two years (17,520 hours) nor over 10 years. The MTBF value we calculated only represents a failure rate that is valid for one year. As the equipment breaks-in or wears out, it most likely will be less and less accurate.
MTBF is not all that useful as we rarely encounter a constant failure rate pattern with equipment. Second, MTBF is just a fancy way of representing a failure rate. IT does provide information on the chance of failure per hour per piece of equipment. IT does not suggest the equipment will have a two-year life with no failures or that the equipment will run for two years with only one failure.
MTBF is not all that helpful for many reasons, one is we often work with people that do not understand what MTBF is or is not. MTBF is not a duration it is a probability of failure, that is all.