Within the scope of the modern workplace, frequency of hazard exposure is relatively agreed-upon. For any given job role, one can analyze what tasks fit into that role, break the tasks down into Job Safety Analyses, and break down the Job Safety Analyses into particular hazards. Those particular hazards can then be assessed for their rates of exposure to employees.
Severity, however, is a little trickier. For example, severity to the individual employee may be considered catastrophic. In healthcare, a back injury from an unfortunate patient movement could literally end a nurse’s career. But in the scope of the entire hospital, while this injury is extremely severe to the nurse, it will not stop the hospital’s operations. For the hospital, this would not be considered severe to the overall scope of operations.