Occurrence Ranking

Occurrence is a relative measure of the frequency of which a failure concept is expected to occur, in either a crisp (1 to 10 ranking) or fuzzy (linguistic e.g. high, very high etc.) value attributed to Faults or Functional Failure Mode. Occurrence categories are based on the RAC / MIL-STD-1629A guidelines.

Occurrence Ranking

The table below shows occurrence rankings and their associated linguistic categories and range values.

Ranking	Category	Value of λ_m (= α×λ_p)	Failure Mode Ratio (α)
10	Frequent	λ_m ≥ 0.2 P_f	> 0.2
7 - 9	Reasonably probable	0.1 P_f ≥ λ_m ≥ 0.2 P_f	0.1 - 0.2
4 - 6	Occasional	0.01 P_f ≥ λ_m ≥ 0.1 P_f	0.01 - 0.1
2 - 3	Remote	0.001 P_f ≥ λ_m ≥ 0.01 P_f	0.001 - 0.01
1	Extremely unlikely	λ_m < 0.001 P_f	< 0.001

Note: The weighting faults or failure modes are set by Failure Mode Ratios (Column 4).

Assignable Rankings	Unassignable Rankings
Faults	Causes
Part-Pair Faults	Mechanisms
Component Faults	Subsystem Failure Modes (FFM)
Part-Pair Failure Modes (FFM)	System Functional Failure Modes (FFM) (Never the originating cause for a failure path.)
Component Functional Failure Modes (FFM)	Symptom/Losses (These do not have a causal effect in the failure path.)

Assigning Occurrence Rankings

Follow the steps below to assign Occurrence Rankings to failure concepts in a model item:

1. Open Failure Diagram editor for a model item or open the Criticality Editor.
2. Select a failure concept in the Failure Diagram or Criticality Editor.
3. Select the Criticality tab in the Properties View.
4. Use the slider bar to change the value or enter your own value in the provided text field.

Note 1: Functional Failure Modes Responses of an item are responses above (high) or below (low) its nominal (zero) response.

Note 2: The FFMs of a source component feed to a target component in the form of an input flow property - this is connected to output flow property of the target item using the causal connection in the Functions editor.