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Design Methods
FIGURE 16.5
Cause and effect diagram.
A roll call method takes attendance to ensure that all members of a group are
present
A duplicate message looks for the receipt of duplicate messages
The FMEA also includes the identification and description of possible
causes for each possible failure mode. Software failure modes are caused by
inherent design faults in the software; therefore, when searching the causes of
postulated failure modes, the design process should be looked at. IEC 60812
gives a table of possible failure causes, which largely are also applicable for
software.
3. Potential failure effects(s): A potential effect is the consequence of the failure on
other entities, as experienced by the user. The relation between effects and their
causes usually is documented in a cause-and-effect (fishbone diagram/ishikawa
diagram) diagram similar to the one depicted in Figure 16.5.
4. Severity: Severity is a subjective measure of “how bad” or “how serious” is the
effect of the failure mode. Usually severity is rated on a discrete scale from 1
(no effect) to 10 (hazardous effect). Severity ratings of 9 or higher (4 or higher
on 5 scale) indicate a potential special effect that needs more attention, and this
typically is a safety or government regulation issue (Table 15.3 is reproduced
as Table 16.2). Severe effects usually are classified as “catastrophic,” “serious,”
“critical,” “marginal,” and “negligible.” “Catastrophic” effects are usually a
safety issue and require deeper study for all causes to the lowest level, possibly
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