Page 360 - The Handbook for Quality Management a Complete Guide to Operational Excellence

346 C o n t i n u o u s I m p r o v e m e n t I m p r o v e / D e s i g n S t a g e 347

Probability of Failure Failure Rate Cpk Occ.
Very high > 1/2 < 0.33 10
1/3 0.33 9
High (often) 1/8 0.51 8
1/20 0.67 7
Moderate; occasional 1/80 .83 6
1/400 1.00 5
1/2,000 1.17 4
Low 1/15,000 1.33 3
Very low 1/150,000 1.50 2
Remote < 1/150,000 > 1.67 1

(AIAG, 1995)
Table 16.2 AIAG Occurrence Levels

8. Define the detection method and likelihood of detection. Table 16.1
provides useful descriptions of detection levels from 1 to 10. In the
example, the failure mode of mistyping the product ID, with the
effect of shipping the wrong product, is given a detectability level
of 4, a likely detection before reaching the customer. This is based
on the detection method that has been implemented from past
process improvements: the accounting clerk compares the PO with
the order form as the invoice is created for shipping.
9. Calculate risk priority number (RPN) by multiplying the severity,
occurrence, and detectability levels. In the example, the risk
priority number is calculated by multiplying 6 (the severity) by 5
(the occurrence level) by 4 (the detection level), resulting in an
RPN of 120.
10. Prioritize the failure modes based on the RPN.

The risk priority number will range from 1 to 1000, with larger num­
bers representing higher risks. Failure modes with higher RPN should be
given priority for the Improve stage of DMAIC.
Some organizations use threshold values, above which preventive
action must be taken. For example, the organization may require improve­
ment for any RPN exceeding 120. Reducing the RPN requires a reduction
in the severity, occurrence, and/or detectability levels associated with the
failure mode. As a general rule:

• Reducing severity requires a change to the design of the product or
process. For example, if the process involves a manufactured part, it
may be possible to alter the design of the part so that the stated
failure mode is no longer a serious problem for the customer.

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