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