Risk Priority Number (RPN

Risk Priority Number (RPN): The risk priority number is the product of the severity (S), occurrence (O),and detection (D) rankings.

(S) x (O) x(D) =RPN

The value is between 1 and 1000 can be used to rank order the concerns in the process.
Engineering assessment for preventive/corrective action should be first directed at high severity, high RPN, and other items designated by the team.

The intent of any recommended action is to reduce rankings in the following order: severity, occurrence , and detection.

In general practice, when the severity is 9 or 10, special attention must be given to ensure that the risk is addressed through existing design actions/controls or process preventive/corrective action(s),regardless of the RPN. In all cases where the effect of an identified potential failure could be a hazard to manufacturing/assembly personnel, preventive/corrective actions should be taken to avoid the failure by eliminating or controlling the cause(s), or appropriate operator protection should b e specified.

After special attention has been given to severity rankings of 9 or 10,the team then addresses other failure, with the intent of reducing severity, then occurrence, and then detection.
Actions such as, but not limited to, the following should be considered:

1-To reduce the probability of occurrence, process and/or design revisions are required. An action-oriented study of the process using statistical methods could be implemented with an ongoing feedback of information to the appropriate operations for continuous improvement and defect prevention.

2-Only a design and/or process revision can bring about a reduction in the severity ranking.

3-The preferred method to accomplish a reduction in the detection ranking is the use of error/mistake proofing methods. Generally , improving detection controls is costly and ineffective for quality improvements. Emphasis must, however, be placed on preventing defects rather than detecting them. An example would be the use of statistical control and process improvement rather than random quality checks or associated inspection.



Articles from FMEA Third Edition DaimlerChrysler Corporation, Ford Motor Company, General Motors Corporation.

How to categorize Process Failure by Severity Ranking

Table1.Suggested Process Failure’s Severity Evaluation Criteria

Effect	Criteria: Severity of Effect This ranking results when a potential failure results in a final customer and/or a manufacturing/assembly plant defect. The final customer should always be considered first. If both occur,use the higher of the two severities. (Customer Effect)	Criteria: Severity of Effect This ranking results when a potential failure results in a final customer and/or a manufacturing/assembly plant defect. The final customer should always be considered first. If both occur,use the higher of the two severities. (Manufacturing / Assembly Effect )	Ranking
Hazardous without warning	Very high severity ranking when a potential failure affects safe product operation and/or involves noncompliance with government regulation without warning	Or may endanger operator (machine or assembly ) without warning	10
Hazardous with warnings	Very high severity ranking when a potential failure affects safe product operation and/or involves noncompliance with government regulation with warning	Or may endanger operator (machine or assembly) with warning	9
Very High	Product or item inoperable (loss of primary function)	Or 100% of product may have to be scrapped, or product / item repaired in repair department with a repair time greater than one hour.	8
High	Product or item operable but at a reduced level of performance. Customer very dissatisfied	Or product may have to be sorted and a portion (less than 100%) scrapped, or product/item repaired in repair department with a repair time between a half-hour and an hour	7
Moderate	Product or item operable but Comfort/Convenience item(s) inoperable. Customer dissatisfied.	Or a portion(less than100%) of the product may have to scrapped with no sorting,or product/item repaired in repair department with a repair time less than a half-hour.	6
Low	Product or item operable but Comfort/Convenience item(s) operable at a reduced level of performance.	Or 100% of product may have to be reworked, or product/item repaired off-line but does not go to repair department.	5
Very Low	Fit and Finish/Squeak and Rattle item does not conform. Defect noticed by most customers (greater than 75%)	Or the product may have to be sorted, with no scrap, and a portion(less than 100%) reworked.	4
Minor	Fit and Finish/Squeak and Rattle item does not conform. Defect noticed by 50% of customers.	Or a portion (less than 100%) of the product may have to be reworked, with no scrap, on-line but out-of-station	3
Very Minor	Fit and Finish/Squeak and Rattle item does not conform. Defect noticed by discriminating customers (less than 25%).	Or a portion (less than 100%) of the product may have to be reworked, with no scrap, on-line but in-station.	2
None	No discernible effect	Or slight inconvenience to operation or operator, or no effect.	1

Note : It is not recommended to modify criteria for ranking values of 9 and 10.Failure with a rank of severity 1 should not be analyzed further.

Severity (S) : Severity is the rank associated with the most serious effect for a given failure. A reduction in severity ranking index can be effected through a design change to system,subsystem or component,or a redesign of the process.

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Detection

Detection (D) : Detection is the rank associated with the best detection control listed in the process control. In order to achieve a lower ranking, generally the planned process control has to be improved.

Assumed the failure has occurred and then assess the capabilities of all “Current Process Controls” to prevent shipment of the part having this failure or defect. Do not automatically presume that the detection ranking is low because the occurrence is low, but do assess the ability of the process controls to detect low frequency failure or prevent them from going further in the process.

Random quality checks are unlikely to detect the existence of an isolated defect and should not influence the detection ranking. Sampling done on statistical basis is a valid detection control.

Table 3. Suggested Process Failure’s Detection Evaluation Criteria

Detection	Criteria	Inspection Types			Suggested Range of Detection Methods	Ranking
		A	B	C
Almost Impossible	Absolute certainty of non detection			X	Cannot detect or is not checked	10
Very Remote	Controls will probably not detect			X	Control is achieved with indirect or random checks only	9
Remote	Controls have poor chance of detection.			X	Control is achieved with visual inspection only.	8
Very Low	Controls have poor chance of detection.			X	Control is achieved with double visual inspection only.	7
Low	Controls may detect		X	X	Control is achieved with charting methods, such as SPC.	6
Moderate	Controls may detect		X		Control is based on variable gauging after parts have left the station, or Go/No Go gauging performed on 100% of the parts after parts have left the station.	5
Moderately High	Controls have a good chance to detect	X	X		Error detection in subsequent operations, OR gauging performed on setup and first-piece check (for set-up causes only)	4
High	Controls have a good chance to detect	X	X		Error detection in-station, or error detection in subsequent operations by multiple layers of acceptance: supply, select, install, verify. Cannot accept discrepant part.	3
Very High	Controls almost certain to detect.	X	X		Error detection in-station (automatic gauging with automatic stop feature).Cannot pass discrepant part.	2
Very High	Controls certain to detect.	X			Discrepant parts cannot be made because item has been error-proofed by process/product design.	1

Inspection Types :

A . Error-proofed

B. Gauging

C. Manual Inspection

Articles from FMEA Third Edition DaimlerChrysler Corporation, Ford Motor Company, General Motors Corporation.