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Item Criticality Number

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When one item under analysis has multiple failure modes and a variety of effects, more than one criticality rating and severity classification may apply. To establish priorities for corrective action for individual items, an item criticality number is calculated for each item within the severity classification. It is the sum of the failure mode criticality numbers within the severity classification. The formula for the item criticality number is:

Cr =

j Σ n = 1

(αKaKeλptβ)n

Nomenclature

symbol	description
j	total number of failure modes for component
n	specific failure mode
α	failure mode ratio
Ka	Operational failure rate modifier
Ke	Environmental failure rate modifier
λp	part failure rate
t	specified time interval
β	failure effect probability