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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
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