“Leading metrics are for indicators which look at conditions that could be expected to contribute to failures, seeking to identify weaknesses before such failures occur. Leading SIS metrics might include quantity and duration of SIF bypasses, quantity and duration of overdue proof-tests, mean time to repair or restore, and many others.”
Reference: Adapted from Barnard, G.S., Safety Instrumented Systems, in Handbook of Loss Prevention Engineering. 2013, Wiley-VCH Verlag GmbH & Co. KGaA. p. 590.