Severity and Frequency

When evaluating hazards of a new job or task many professionals evaluate severity versus frequency (or likelihood of occurring). Using this method enables an individual to compare a less severe butmore frequent outcome to a severe but less frequently occurring outcome to decide where to place the most time, energy, and resources in mitigating hazards to prevent incidents.


Severity is defined as “the fact or condition of being severe”. Severe is then defined as “of something bad or undesirable; intense”. We are all familiar with terms such as severe storm or severe consequences, but severe can mean different things to different people. When evaluating outcomes of an incident occurring it is useful to use a scale from 1 to 10 or 1 to 100 and assign examples to the numbers to help define severity. For example, a person who is evaluating the negative outcomes created by a hazard of a new work task may feel that an incident that results in a recordable injury is a 7 on a 1 to 10 scale. A 10 on the scale may be a fatality or losing an entire building to a fire. This creates more of a solid definition of what is considered severe to the individual, group, or company evaluating the hazards and negative outcomes of a new work task.


Frequency is defined as “the rate at which something occurs or is repeated over a particular period of time or in a given sample”. The term frequent, just like severe, can also mean many different things to different people. The same method of assigning numbers to severity can be done for frequency to better define it. After defining frequency, you can look at both severity and frequency of negative outcomes resulting from the different hazards of a new work task or a possible negative event.

Why Both Severity and Frequency Need to be Evaluated:

It is not enough to just focus on the hazards or events that have the most severe outcomes. If the frequency of the event causing the severe outcome is very low, then it may not be what needs the most attention. A hazard or event that occurs very frequently and causes a low severity incident may need more attention to protect employees.


Defining and looking at both severity and frequency helps to avoid only focusing on the less frequent “big” hazards that result in severe consequences such as a fatality. Attention also needs to be given to issues such as trip hazards that are much more frequent with a lesser outcome of severity. Often times it is smaller more common hazards that lead to the majority of workplace injuries, not the hazards that cause the most severe outcomes.

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