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116 Safety Risk Management for Medical Devices
Table 12.14 Definitions of UMFMEA detectability ratings
Detection Criteria (Det)
Category Rank Qualitative criteria
Undetectable 5 Effect is not immediately visible or knowable | Countermeasures
not possible
Low 4 Effect can be visible or knowable only with expert investigation
using specialized equipment | Countermeasures are unlikely
Moderate 3 Effect can be visible or knowable with the moderate effort by user |
Countermeasures are probable
High 2 Highly Detectable—Effect can be visible or knowable with simple
action by user, from the information provided by the system itself
| Countermeasures are likely
Almost 1 Almost certain detection—Effect is clearly visible or knowable to
Certain user without any further action by user | Countermeasures are
certain
implantable device could result in the implantation of a contaminated device, which
could lead to infection. In the example HAL seen in Fig. 12.13, Infection has the
highest probability of causing a Serious Harm (72.1%). Therefore in this example you
would choose the severity ranking of 3, Serious. Remember that this ranking is used
only for criticality determination and the prioritization of resources on mitigating
Failure Modes. The safety risk of Hazards is computed in the RACT and is a different
matter.
Occurrence ranking signifies the estimate of the highest likelihood of occurrence
of a use failure. The Standard [14] indicates that quantitative estimation of
Occurrence should not be made unless you have data to support your estimation. In
the absence of data, you can make qualitative estimates of the potential for a use fail-
ure based on the descriptions in Table 12.13.
Det, or Detectability, is the likelihood of detection of the use failure. Detection
may occur anywhere in the causal chain, from the initial use error, to an intermediate
effect, to the final End Effect at the System level. Use Table 12.14 to select a ranking
for Detectability. Important: consider the ability of the user to take countermeasures to prevent
the Hazardous Situation, or take actions to minimize Harm severity. Detection of the End
Effect where there is nothing that the user can do to minimize the risk of Harm is of
little value. In such cases, choose a ranking of 5.
RPN is a measure of criticality of a Failure Mode. RPN is the product of the
rankings of Severity, Occurrence, and Detection. This number is used to decide the