Application to FMEA Priority Assessments - Probabilistic Composition of Preferences, Theory and Applications - page 112

11.3 Risk Indices

According to each of the criteria of severity, occurrence and detectability, the
modes of failure are classi ed into a certain number of levels, properly determined

previously. The previous description of levels based on the reality of the production
process is one of the most important recommendations for FMEA (Stamatis 1995 ;
Pfeifer 2002 ). Tables 11.1 , 11.2 and 11.3 below provide an example of generic
descriptions for five levels indices.
The construction of the tables of identi

cation of the levels and the association to
each mode of failure of a description in each table is an approximate process. Bad
results in the application of the technique are frequently associated to errors in the
construction and use of the scales. For this reason is important to take into account
the uncertainty inherent to the evaluations.
Uncertainty is common in risk evaluations. It is expected that the available data
are subject to fluctuations in their values if measurements are taken in different

instances. When the preferences, as in the case of assigning priorities to possible

Table 11.1 Levels of
severity

Level Importance Characterization

1 Very low The use will not care

2 Low Light loss of performance

3 Moderate Real source of dissatisfaction

4 High Inoperative system

5 Very high Inoperative system menacing security

Table 11.2 Levels of
undetectability

Level Importance Characterization

1 Very low Certainly will be detected

2 Low Most possibly detected

3 Moderate Median chance of passing undetected

4 High Most possibly undetected

5 Very high Certainly will not be detected

Table 11.3 Levels of
occurrence

Level Importance Characterization

1 Very low 1 in 1.000.000

2 Low 1 in 10.000

3 Moderate 1 in 1.000

4 High 1 in 1.00

5 Very high 1 in 10