Mean Time Between Failures
This value can be found on the technical specs sheet from several hardware components and indicates the mean time between failures that occurred with that specific product in its manufacturer labs. This time is usually given in hours.
MTBF calculation is doing like this. The manufacturer defines the test procedure by defining the number of components that will be tested at the same time and the number of hours that the test will run. Multiplying the two we have the Total Power-On Hours (TPOH). The TPOH is then divided by the number of defective components found during the test period. For example, if a hard disk drive manufacturer tests 1,000 disk during 30 days (720 hours) e only one hard disk drive was defective during this period, MTBF will be of 720,000 hours (1,000 disks x 720 hours / 1 defective unit).
There are two main wrong assumptions regarding MTBF that must be clarified.
First, it is not a reliability measurement unit. If it was, the hard disk drives from the example would last 720,000 hours (82 years of non-stop usage). Which we know that is not true.
Second, it is not possible to compare the MTBF from a product with the MTBF from another product. The reason is simple: each manufacturer uses its own methodology (number of parts tests at the same time and number of hours on which the test is run).