Page 120 - Safety Risk Management for Medical Devices

Risk Analysis Techniques 99

To rank the severity of End Effects without a safety impact, it’s necessary to know
the primary and secondary functions of the item under analysis. For example, if a
Failure Mode causes the complete loss of the primary functions, the severity ranking
would be 4, Critical.
To rank the severity of an End Effect that has a safety impact, consider the effect at
the System level. That is because to receive the benefit of a medical device, the users/
patients interact with the System, not just the individual components of the System.

Table 12.4 Definitions of DFMEA Occurrence Ratings
Probability of Occurrence Criteria (Occ)
Category Rank Qualitative criteria Quantitative
criteria

Frequent 5 The occurrence is frequent. Failure may be almost $ 10 23
certain or constant failure
23
Probable 4 The occurrence is probable. Failure may be likely | ,10 and
repeated failures are expected $ 10 24
24
Occasional 3 The occurrence is occasional. Failures may occur at ,10 and
infrequent intervals $ 10 25
Remote 2 The occurrence is remote. Failures are seldom ,10 25 and
expected to occur $ 10 26
Improbable 1 The occurrence is improbable. The failure is not ,10 26
expected to occur

Table 12.5 DFMEA detectability ratings
Detection Criteria (Det)
Category Rank Qualitative criteria Quantitative
criteria
3
Undetectable 5 No detection opportunity | No means for ,10
detection | Countermeasures not possible
Low 4 Opportunity for detection is low | ,10 2 and
Countermeasures are unlikely $ 10 3
Moderate 3 Opportunity for detection is moderate | ,10 1 and
Countermeasures are probable $ 10 2
High 2 Opportunity for detection is high | , 9 3 10 21
21
Countermeasures are likely and $ 10
Almost 1 Opportunity for detection is almost certain | $ 9x10 21
Certain Countermeasures are certain