Like most of the JHA process, there is some subjectivity involved. But certainly, the number of employees exposed to the hazard is a good indicator of probability of something happening. Also, it’s important to look at the duration of exposure: length of time an employee is on a ladder, for example.
And, you can certainly look at how close the worker is to whatever the hazard is. For example, are they at arm’s length to the machine hazard? Are they within the breathing zone to the chemical? At body’s length from the fall hazard? You can also factor in the pace of the work, both in terms of ergonomics issues and also just general hazard exposure.
Finally, environmental factors, such as the temperature, noise, or weather, can come into play.