In reliability engineering, what does FIT stand for, and how is MTBF related to FIT when hazard is constant?

• A. FIT stands for Failures In Time; MTBF = 10^9 / FIT
• B. FIT stands for Failures In Time; MTBF = 10^9 × FIT
• C. FIT stands for Failures In Time; MTBF = FIT / 10^9
• D. FIT stands for Failures In Time; MTBF = 10^9 / (FIT × 2)

Answer: (A)

FIT stands for Failures In Time, a rate expressed as failures per 1e9 hours of operation. If the hazard is constant, MTBF is the reciprocal of the hazard. Since lambda = FIT / 1e9, the mean time between failures is MTBF = 1 / lambda = 1 / (FIT / 1e9) = 1e9 / FIT hours. This shows the inverse relationship: as FIT grows, MTBF shrinks. The 1e9 factor ensures the units line up correctly to give MTBF in hours.