Calculating the RPN

What is the RPN?

So we have identified a number of risks, to calculate and allocate the Risk Priority Number (RPN) we have a number of tools that we can use – these can be found along with how to use them by a simple search.

However I would like to take you through one particular tool, the Failure Mode and Effect Analysis (FMEA). This tool works well for most things, as a tool it does have some limitations – but it is very good for evaluating risk.

As with all the other tools suitable for evaluating risk, the end result is the RPN. How does it work, simply based on user experience and expectations – it will provide a value that you assign to the risk.

This value is based on three things, they are;

1. The Severity of the Risk (SEV)
2. The Likelihood of Occurrence (OCC)
3. The Likelihood of Detection (DET)

Quite simply these three things are assigned values, usually in the range of one to ten and a multiplicand is produced – this is the RPN.

What do the numbers mean?

The numbers assigned to Severity, Occurrence and Detection are arbitrary and people do tend to have their own interpretations on the values as a rule of thumb I tend to use the following – you could use any range of numbers that suits yor requirements.

Severity.

1. Insignificant Impact
2. Minor Impact
3. Significant Impact
4. Major Impact
5. Catastrophic Impact

Occurence.

1. Unlikely
2. Possible
3. Likely
4. Probable
5. Certain

 Detection.

1. Certain
2. Probable
3. Likely
4. Possible
5. Unlikely

As can be seen the Detection values run contrary to the other two (Severity and Occurence), I find that it is good to have the list of whatever values you feel should be assigned to hand during the FMEA.