Give each measure a score. My suggestion? Start simple. You can always add more complexity or sophistication later, if you really need to. It’s much harder to move in the opposite direction.

Likelihood

• extremely unlikely = 1
• unlikely = 2
• likely = 3
• almost certain = 4

 

Impact

• immaterial = 1
• material = 2
• critical = 3
• catastrophic = 4

 

Multiply score for likelihood with score for impact, giving you a score for severity that can range from 1 to 16. Higher score? Higher priority.

Simples!