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Chapter 19 Risk management 585
Table 19.2 Rating scales for FMEA
A. Occurrence of failure
Description Rating Possible failure
occurrence
Remote probability of occurrence 1 0
It would be unreasonable to expect failure to occur
Low probability of occurrence 2 1:20,000
Generally associated with activities similar to
previous ones with a relatively low number of failures 3 1:10,000
Moderate probability of occurrence 4 1:2,000
Generally associated with activities similar to previous 5 1:1,000
ones which have resulted in occasional failures 6 1:200
High probability of occurrence 7 1:100
Generally associated with activities similar
to ones which have traditionally caused problems 8 1:20
Very high probability of occurrence 9 1:10
Near certainty that major failures will occur 10 1:2
B. Severity of failure
Description
Rating
Minor severity 1
A very minor failure which would have no noticeable effect
on system performance
Low severity 2
A minor failure causing only slight customer annoyance 3
Moderate severity 4
A failure which would cause some customer dissatisfaction, 5
discomfort or annoyance, or would cause noticeable 6
deterioration in performance
High severity 7
A failure which would engender a high degree of 8
customer dissatisfaction
Very high severity 9
A failure which would affect safety
Catastrophic 10
A failure which may cause damage to property,
serious injury or death
C. Detection of failure
Description Rating Probability of
detection
Remote probability that the defect will reach the customer 1 0 to 15%
(It is unlikely that such a defect would pass through
inspection, test or assembly)
Low probability that the defect will reach 2 6 to 15%
the customer 3 16 to 25%
Moderate probability that the defect will 4 26 to 35%
reach the customer 5 36 to 45%
6 46 to 55%
High probability that the defect will reach 7 56 to 65%
the customer 8 66 to 75%
Very high probability that the defect will 9 76 to 85%
reach the customer 10 86 to 100%