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Occurrence uses percentage-based probability ranges, from Never (<5%) to Always (>95%), with a dedicated Unanalyzed (0) level for tracking incomplete assessments.

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Occurrence Scale

RatingIDNameProbability RangeDescription
00Unanalyzed—Default. Failure mode occurrence has not yet been assessed.
11Never< 5%Failure mode is highly improbable under expected conditions
22Seldom5% — 25%Failure mode may occur in a small fraction of units or uses
33Sometimes25% — 75%Failure mode occurs in a significant portion of units or uses
44Often75% — 95%Failure mode is expected in most units or uses
55Always> 95%Failure mode occurs in virtually every unit or use
Failure modes default to occurrence = 0 (Unanalyzed), explicitly marking items that have not been assessed. Since RPN = S x 0 x D = 0, unanalyzed items will have an RPN of zero and should be reviewed during FMEA completion checks.

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Fields Using This Enumeration

The fmOccurrence enum is used on two Failure Mode fields, representing the before/after states of risk mitigation:
FieldIDStage
Pre-mitigation occurrencepremitigationFMOccurrenceInitial FMEA rating — before design changes or process controls
Post-mitigation occurrencepostmitigationFMOccurrenceResidual FMEA rating — after corrective actions are implemented

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Role in RPN Calculation

Occurrence is one of three factors in the Risk Priority Number: 
         Severity (S)     Occurrence (O)     Detection (D)
            1-5      x       1-5        x       1-5       =  RPN (1-125)

     How severe is     How often does      How likely is the
     the effect?       the failure occur?  failure detected?
Example: A failure mode with Severity=4 (High), Occurrence=3 (Sometimes), Detection=2 (Seldom detected) yields: 
RPN = 4 x 3 x 2 = 24
After implementing a design change that reduces occurrence to 1 (Never): 
Post-RPN = 4 x 1 x 2 = 8   (67% reduction)

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Cascade Mapping: SubType to Occurrence

The solution includes a cascade mapping (subType_-__fmOccurrence) that can filter available occurrence values based on the design requirement subtype. This enables contextual FMEA scoring where the valid occurrence range depends on the engineering discipline:
Design Requirement SubTypeAvailable Occurrence Values
OperationFiltered set per configuration
SoftwareFiltered set per configuration
DesignFiltered set per configuration
UseFiltered set per configuration
SystemFiltered set per configuration
ProcessFiltered set per configuration
The subtype-to-occurrence cascade is a many-to-many mapping — each subtype can map to multiple occurrence levels. This acts as a filter rather than a restriction, guiding assessors toward appropriate occurrence ratings for their discipline.

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Comparison: FM Occurrence vs. P1 Hazard Probability

PropertyFM OccurrenceP1 Hazard Probability
Used inDFMEA / PFMEAHARA
Scale0-5 (percentage-based)0-5 (frequency-based)
MeasuresHow often the failure mode occursHow often the hazardous situation occurs
Pre-mitigation fieldpremitigationFMOccurrencepreHazardProbability
Post-mitigation fieldpostmitigationFMOccurrencepostHazardProbability
Role in risk calculationO factor in RPN = S x O x DP1 factor in P = bucket(P1 x P2)

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Configuration Details

PropertyValue
Source file.polarion/tracker/fields/failureMode-fmOccurrence-enum.xml
Enum typefmOccurrence
Applied toFailure Mode (premitigationFMOccurrence, postmitigationFMOccurrence)
Cascade mappingsubType_-__fmOccurrence (in mappings.xml)
Used in reportsDFMEA Summary Report
Code: failureMode-fmOccurrence-enum.xml, failureMode-custom-fields.xml, mappings.xml