What Does Mean Time Between Failures (MTBF) Mean?
Mean time between failure (MTBF) refers to the average amount of time that a device or product functions before failing. This unit of measurement includes only operational time between failures and does not include repair times, assuming the item is repaired and begins functioning again. MTBF figures are often used to project how likely a single unit is to fail within a certain period of time.
Techopedia Explains Mean Time Between Failures (MTBF)
One import aspect of MTBF is that those looking at these kinds of statistics should know whether the measurement applies to one unit that functions until failure, or a large number of different units run for a short time, where the MTBF represents the likelihood of failure during this shorter testing phase. By itself, MTBF does not always indicate how long the test process is for a given unit. For example, if a thousand devices are run for several hours each and 1 percent of them malfunction, this will yield different results than if one unit is tested until it eventually fails.