Going through the mathematics can reveal interesting realities. The failures for a constant failure rate are characterized by an exponential factor, so only 37% of the units in large group will last as long as the MTBF number. Second, for a single supply, the probability that it will last as long as its MTBF rating is only 37%. Third, there is a 37% confidence level likelihood that it will last as long as its MTBF rating. Additionally, half the components in a group will have failed after just 0.69 of the MTTF.