Learn in the textbookReliability Metrics
DefinitionThe failure rate (故障率) measures how many times a device or system fails per unit of time.
ExplanationA smaller value means failures are less likely. When failures can be treated as occurring at a roughly steady pace regardless of time, the failure rate is the reciprocal of MTBF (mean time between failures, the next term), so an MTBF of 400 hours gives a failure rate of 0.0025 failures per hour. In real equipment the failure rate shifts between first use and the end of its life, and the curve tracing that shift is called the bathtub curve (バスタブ曲線).
AnalogyIt is like counting how many flat tires your commuter bike gets in a year. The fewer flats, the lower its failure rate.
Memory hookRemember it by the units: failure rate is "failures per hour" and MTBF is "hours per failure", the same fraction flipped upside down.