Which statement best differentiates qualitative and quantitative risk assessment in process safety?

• A. Qualitative risk assessment uses descriptive scales to rank hazards; quantitative risk assessment assigns numerical probabilities and consequences to calculate numerical risk.
• B. Qualitative risk assessment uses descriptive scales to rank hazards; quantitative risk assessment uses numerical probabilities.
• C. Qualitative risk assessment uses only historical data; quantitative risk assessment uses theoretical models.
• D. Qualitative and quantitative assessments always yield identical results.

Answer: (B)

Understanding how risk is expressed helps you differentiate qualitative from quantitative assessments. In qualitative risk assessment, risk is described with descriptive scales such as low, medium, or high to rank hazards and prioritize actions. There’s no exact numerical value attached to probabilities or consequences, which makes it useful when data are limited or quick decisions are needed.

In quantitative risk assessment, numerical probabilities are estimated for initiating events and numerical values are assigned to consequences. These numbers are then combined to produce a numerical risk metric, such as expected fatalities per year or risk in monetary terms. This allows objective comparisons and more precise prioritization based on quantified estimates.

The statement that best differentiates them is that qualitative risk assessment uses descriptive scales to rank hazards, while quantitative risk assessment uses numerical probabilities to calculate numerical risk. For context, a qualitative approach might say a release is high risk because the consequences are severe and the likelihood is moderate, whereas a quantitative analysis would provide a numeric probability of the release and a numeric estimate of potential impacts to compute a specific risk value. The other descriptions do not accurately capture how these methods operate.