At the Westgard web site, the pros and cons of dropping detection from RPN (severity, probability of occurrence, and probability of detection) are discussed. Rather than comment on an abstract level, here is an example of why I favor not including detection.
Assume potassium is being analyzed with the potential for an erroneously high potassium result to be reported to the clinician. This sets the severity to be the same for all causes of this event. There are many, many potential failure causes which require a ranking to focus on mitigations for the top causes that have unacceptable risk. Since all causes have the same severity, and since I don’t favor using probability of detection, it remains to determine the probability of occurrence for each failure mode.
Potential failure (= error) causes are postulated for each process step, of which there are many process steps in a detailed process map. Let’s consider one process step and one potential error cause. Additional process steps deal with this error.
Sample process step – Sample processed to provide serum. A potential error in this step is cell hemolysis.
Detection process step – Technician examines sample for hemolysis.
Recovery process step – If hemolysis has been detected, technician prevents the sample from being analyzed.
For each of these process steps, one must estimate the probability of occurrence for a failure cause. Moreover, each process step can have multiple failure causes. For the sample process step, one could perhaps tally the percentage of detected hemolyzed samples. It is important to understand that both the detection and recovery steps can also fail and their probability of occurrence of failure must also be estimated. These potential failure rates would seem to be harder to estimate although each would have as a floor, non cognitive error.
Given all process step failure modes have been ranked in descending order by probability of occurrence, one would consider mitigations for those process step errors with unacceptable risk.
I don’t see how the addition of classifying probability of detection would help.