Quality Progress - May 2017 - 41

Although visual inspection and random quality checks
have their purposes, these forms of process control have
limited impact on the Det ranking for risk assessment. Neither can detect the mechanism of failure to systematically
prevent all failures.
When defects are created, eventually some will find a
way to escape the operation and reach the customer. When
visual inspection acts as the only method of Det, the probability of detecting failures before they leave the operation
remains limited.
Adding a third or a fourth human visual inspector, for
example, might not actually improve the probability that
visual inspection alone will detect significantly more failures. Random quality checks are unlikely to detect isolated
defects and should not influence the Det ranking.
A potential effect of an inaccurate RPN value is unmitigated risk because an FMEA team might underestimate
particular key risks and fail to take appropriate action.
Some organizations still employ an arbitrary RPN threshold
value beyond which action is required and below which no
action is needed.
It is important to note that recent industry thought
leadership does not recommend the use of an RPN threshold to initiate the need for action. Inaccurate RPN values
below an arbitrary action threshold might lead teams to be

complacent with particular failure modes.
A marginal error in Sev, Occ or Det might Driver distraction
significantly decrease the RPN, perhaps
occurs much more
decreasing its value below an action
often than many
threshold value. Table 3 provides an illuswant to admit.
trative example of how input ranking error
might affect the RPN.
The example uses an arbitrary RPN action threshold
value of 100 and a scenario in which Sev = 10, Occ = 8, and
an inaccurate ranking of Det = 1. This results in an RPN value
of 80 (= 10 × 8 × 1). Because 80 is less than 100, the team
might defend a decision for no action.
Consider the possibility the inaccurate Det ranking
requires a marginal correction of plus one (+1) unit. By
substituting Det = 2 instead of Det = 1, the RPN value
changes from 80 to 160 (= 10 × 8 × 2). This change is further
illustrated in Table 4.
Note that this particular correction causes a 100%
increase in the RPN value. Since 160 is greater than 100,
action now would be required by the team. This is the
opposite of the previous decision.

RPN sensitivity

It is important to recognize that some RPN values are more
sensitive to input ranking errors than others. Sensitivity

TA B L E   5

TA B L E 4

Graphical example of how
potential marginal error
affects RPN value
Original

Marginal change

Partially completed RSPN/RPN
matrix for Sev = 10
10

200/100

300/200

400/300

500/400

600/500

9

180/90

270/180

360/270

450/360

540/450

8

160/80

240/160

320/240

400/320

480/400

140/70

210/140

280/210

350/280

420/350

Sev

10

10

7

Occ

8

8

6

120/60

180/120

240/180

300/240

360/300

Det

1

2

5

100/50

150/100

200/150

250/200

300/250

RPN

80

160

4

80/40

120/80

160/120

200/160

3

60/30

90/60

120/90

2

40/20

60/40

1

20/10

Error +1
RSPN

Min
 1

100

240

Sev = severity
Det = detection
Occ = occurrence
Min = minimum
RPN = risk priority number
RSPN = risk sensitive priority number

1

2

3

4

5

RSPN/RPN RSPN > 100/RPN < 100
RPN = risk priority number
RSPN = risk sensitive priority number

qualityprogress.com ❘ May 2017

QP 41


http://www.qualityprogress.com

Table of Contents for the Digital Edition of Quality Progress - May 2017

Seen and Heard
Expert Answers
Progress Report
Mr. Pareto Head
Field Notes
Mind the Gaps
Solid Footing
Are These the Same?
New Tricks for an Old Tool
Innovation Imperative
Experience More
Career Coach
Statistics Spotlight
Standard Issues
Marketplace
Footnotes
Back to Basics
Quality Progress - May 2017 - intro
Quality Progress - May 2017 - cover1
Quality Progress - May 2017 - cover2
Quality Progress - May 2017 - 1
Quality Progress - May 2017 - 2
Quality Progress - May 2017 - 3
Quality Progress - May 2017 - 4
Quality Progress - May 2017 - 5
Quality Progress - May 2017 - Seen and Heard
Quality Progress - May 2017 - Expert Answers
Quality Progress - May 2017 - Progress Report
Quality Progress - May 2017 - Mr. Pareto Head
Quality Progress - May 2017 - 10
Quality Progress - May 2017 - 11
Quality Progress - May 2017 - Field Notes
Quality Progress - May 2017 - 13
Quality Progress - May 2017 - 14
Quality Progress - May 2017 - 15
Quality Progress - May 2017 - Mind the Gaps
Quality Progress - May 2017 - 17
Quality Progress - May 2017 - 18
Quality Progress - May 2017 - 19
Quality Progress - May 2017 - 20
Quality Progress - May 2017 - 21
Quality Progress - May 2017 - 22
Quality Progress - May 2017 - 23
Quality Progress - May 2017 - Solid Footing
Quality Progress - May 2017 - 25
Quality Progress - May 2017 - 26
Quality Progress - May 2017 - 27
Quality Progress - May 2017 - 28
Quality Progress - May 2017 - 29
Quality Progress - May 2017 - Are These the Same?
Quality Progress - May 2017 - 31
Quality Progress - May 2017 - 32
Quality Progress - May 2017 - 33
Quality Progress - May 2017 - 34
Quality Progress - May 2017 - 35
Quality Progress - May 2017 - New Tricks for an Old Tool
Quality Progress - May 2017 - 37
Quality Progress - May 2017 - 38
Quality Progress - May 2017 - 39
Quality Progress - May 2017 - 40
Quality Progress - May 2017 - 41
Quality Progress - May 2017 - 42
Quality Progress - May 2017 - 43
Quality Progress - May 2017 - Innovation Imperative
Quality Progress - May 2017 - 45
Quality Progress - May 2017 - 46
Quality Progress - May 2017 - Career Coach
Quality Progress - May 2017 - 48
Quality Progress - May 2017 - 49
Quality Progress - May 2017 - Statistics Spotlight
Quality Progress - May 2017 - 51
Quality Progress - May 2017 - 52
Quality Progress - May 2017 - 53
Quality Progress - May 2017 - Standard Issues
Quality Progress - May 2017 - 55
Quality Progress - May 2017 - 56
Quality Progress - May 2017 - 57
Quality Progress - May 2017 - Marketplace
Quality Progress - May 2017 - 59
Quality Progress - May 2017 - Footnotes
Quality Progress - May 2017 - 61
Quality Progress - May 2017 - 62
Quality Progress - May 2017 - 63
Quality Progress - May 2017 - Back to Basics
Quality Progress - May 2017 - cover3
Quality Progress - May 2017 - cover4
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