In practical applications of common cause failure (CCF) modelling the beta factor model is often preferred. A problem with the beta factor approach is that for any M-out-of-N (MooN) structure1 of the same type of components (channels), we get the same rate of CCFs. If λ. is the channel failure rate, the common cause failure rate of an MooN configuration is λMOON = β λ for any N ≥ 2 and any 1 ≤ M < N. The beta factor approach does not distinguish between different configurations, and the same result is obtained for a 1002, 1003, and 2003 structure (unless different β’s are used). The new standard, IEC 61508 [1] suggests the use of the beta factor model for quantifying the safety unavailability (i.e. probability of failure on demand, PFD). According to our opinion, this will in most cases not give a satisfactory basis for decisions concerning safety availability.