Maintenance is any form of intervention in a system with the intent either to keep the system in operational state or to restore it to an operational state after failure. Most systems require preventive and corrective maintenance; the latter occurs after failure, while the former is designed to prevent failure and so lengthen the MTTF (mean time to first system failure). Preventive maintenance may include scheduled inspections or condition-based maintenance. The latter is concerned with signs of future failure and identification of any trend in modes of failure. The mean time between failures (MTBF) is to do with the distribution of time between points when a system is restored from a failed to a working state. Steady-state availability is also given by MTBF, where MTBF is the mean time between failures but it should be noted that this value is only approached after a large number of repairs.