Category

Range (failures per year)

Catastrophic

Critical

Marginal

Negligible

Frequent

>10−3

I

I

I

II

Probable

10−3 to 10−4

I

I

II

III

Occasional

10−4 to 10−5

I

II

III

III

Remote

10−5 to 10−6

II

III

III

IV

Improbable

10−6 to 10−7

III

III

IV

IV

Incredible

<10−7

IV

IV

IV

IV