Full definition
Number of failures per unit of operating time (λ = failures/hour). In the useful life period with exponential distribution it is constant; in infant mortality it is decreasing (Weibull β<1); in wear-out it is increasing (Weibull β>1). Relationship with MTBF: λ = 1/MTBF (for exponential distribution). Estimated from CMMS failure history or reliability databases such as OREDA (oil/gas) and IEEE Std 493. Enables calculation of failure probability during a period and sizing of preventive maintenance strategies.