I(可忽略)

II(较轻)

III(严重)

IV(灾难性)

A不可能P < 0.01%

IA

IIA

IIIA

IVA

B难得发生0.01% = P < 0.1%

IB

IIB

IIIB

IVB

C偶尔发生0.1% = P < 1%

IC

IIC

IIIC

IVC

D可能发生1% = P < 10%

ID

IID

IIID

IVD

E频繁发生P ≥ 10%

IE

IIE

IIIE

IVE