How to Assign "Likelihood" in Assessing H&S Risks?!

If you are familiar with risk assessment, you probably have struggled with how to assign the likelihood in the famous risk equation:

Risk = Consequences x Likelihood

According to the IEC/ISO 31010 section 5.3.4 Likelihood analysis and probability estimation the following are the available techniques:

• Use of relevant historical data. In our semi-quantitative risk assessment, this means use of accidents and near misses data.
• Probability forecasts using predictive techniques such as fault tree. In our scope this is not practical. We can't ask each department to do fault tree for each hazardous event expected. We struggle already to motivate them to assess the risk in the first place. A more complex approach is not helpful even if it yields good results.
• Expert opinion using structured approach. Again, this will require tremendous amount of resources. Although necessary for certain types of risk assessments; fire risk assessment, electrical risk assessment... it will not help in semi-quantitative risk assessments the company perform internally for its activities.

The solution for this challenge is to use the Hierarchy of Control Measures to develop a likelihood guidance.

Looking at the above Hierarchy, you can notice that the effectiveness of the controls decreases as you go down the hierarchy. The effectiveness of existing controls can be correlated to the risk equation.

I have developed and implemented the above approach in many situations with good and consistent results.