SOFTWARE FAILURE MODE AND EFFECT ANALYSIS (SFMEA) - Software Design for Six-Sigma: A Roadmap for Excellence - page 428

TABLE 16.4 SFMEA Likelihood of Occurrence

Likelihood of Occurrence Rating

Rating
1-5

Rating
1-10

Criteria Description

Frequent Hazard/Harm likely to occur frequently :1per10

min (1/10) to 1 + per min (1/1)

5 9-10

Probable Hazard/Harm will occur several times during the

life of the software : 1 per shift (1/480) to 1 per hour
(1/60)

4 7-8

Occasional Hazard/Harm likely to occur sometime during the

life of the software : 1 per week (1/10k) to 1 per day
(1/1440)

3 5-6

Remote Hazard/Harm unlikely, but possible to occur during

the life of the software : 1 per 1 unit-year (1/525k) to
1 per 1 unit-month (1/43k)

2 3-4

Improbable Hazard/Harm unlikely to occur during the life of

the software : 1 per 100 unit-years (1/50m) to 1 per
10 unit-years (1/5m)

1 1-2

techniques for failure detection by asking: “In what means they can recognize
the failure mode?” In addition, “how they can discover its occurrence?”
Design controls span a spectrum of different actions that include changes and
upgrades (without creating vulnerabilities), special controls, design guidelines,
DOEs, design verification plans, and modifications of standards, procedures,
and best-practiced guidelines.
7. Detection: Detection is a subjective rating corresponding to the likelihood that
the detection method will detect the first-level failure of a potential failure
mode. This rating is based on the effectiveness of the control system through
related events in the design algorithm; hence, FMEA is a living document. The
DFSS team should:
8. Assess the capability of each detection method and how early in the DFSS
endeavor each method will be used
9. Review all detection methods in column 8 and achieve a consensus on a detec-
tion rating
10. Rate the methods. Select the lowest detection rating in case of a tie.
Examples of detection methods are assertions, code checks on incoming
and outgoing data, and sequence checks on operations. See Table 16.5 for
recommended ratings.
11. The product of severity (column 4), Occurrence (Column 6) and detection
(Column 8) ratings. The range is between 1 and 1,000 (on a 1-10 scale) or
between 1 and 125 (on a 1-5 scale).