|Risk Priority Results|
|Occurance||1||Remote: Unlikely <1,500,001|
|RPN (S * O * D)||1||Of 1000|
|Acceptability||N||Action Not Required|
|The letters and numbers inside the table indicate whether a corrective action is required for each case.
Acceptable Action LevelTo further determine an Acceptable Action Level required for achieving a lower risk to the potential or actual failures, simply subtract the Risk Ranking Table results from the set Detectability Level. To simplify, change the Risk Ranking Table values of C = 1 and N =11.
|Min Req Detectability||11||None Continue Normal Sampling|
|Detectability Offset||10||Above Required Minimum|
|Risk Ranking Table|
This form will help to assess a Risk Priority Number (RPN) for manufactured devices. The RPN is simply number to set a priority of action based on a risk assessment of potential problems identified during a Failure Mode and Effects Analysis (FMEA). The RPN value is determined by:
RPN = S * O * D
- S = Severity of the problem
- O = Occurrence of the problem
- D = Detectability of the problem
Usually a ranking of 1-5 or up to 1-10 is used. Severity and Occurrence Values range from low to high and Detectability is rated high to low (Less Detectability = Higher value). The values set for each rank is a subjective decision based on the criticality of the risk, however once they have been decided upon they become the standard for the assessment process.
The calculated RPN value can vary depending on the range of the ranking values. If ranking of 1-5 is used for S, O and D then the value will be from 1 to (5 * 5 * 5) or 125. For the ranking of 1-10 this becomes 1 to (10 * 10 * 10) or 1000. This gives you an overall Priority Number based on S * O * D. The higher the RPN number the higher the risk no matter the range.
The next step of the Risk Assessment Evaluation uses a Risk Ranking Table to determine the appropriate action based upon the O, S and D rankings. As an example, the the table below consists of 10 Columns(Severity) with 10 Rows(Occurrence). Using the S and O values we refer to the table to select the “Risk Ranking” values. 3 different courses of action are suggested. For an ranking of “N” No action is required, For a ranking of “C” Then Action is required. All other ranking values between 1 and 10 indicate a level of urgency and require further investigation or action. To further determine action or investigation requirement, subtract the selected Detectability Level from the S and O Risk Ranking Table value. A number of less than 0 indicates that further action should be taken. A number of 0 or above indicates that your detectability is adequate to reduce the risk, however further investigation should continue.
The main object of the RPN is only to set a priority based on a standard set of risk assumptions. When using RPN’s to determine risk and set priorities, it is important to remember that a low or “N” rating does not mean that the failure is acceptable. it is just a lower priority to the higher or “C” rating. Many low priority values can be an accumulated to cause an overall higher risk. In other words many small failure modes can be just as detrimental as one or two large ones. So don’t ignore the little problems.
While this provides a good method for the study of perceived or potential problems in a FMEA or pFMEA project, I’ve found it useful in Process Controls along with paretos and other SPC methods to determine where actions need to be taken. What is does is help to create a structured action path that management agrees upon allowing for quicker resolution of non conformance’s. This also becomes quite useful during an ISO audit if used as part of the corrective/preventative action process.