Beginners Guide : Difference between of Probability and severity - Must Read article .


Difference between probability and severity ??


One of the frequent and often asked interview questions in safety is that, the difference between probability and severity.

The probability and severity are a most important elements involved in risk.


Simply let me explain you with the three scenarios,




Understanding part 1:


Situation 1: Driving a car which is in good condition and properly maintained.

Situation 2: Driving a car which is poorly maintained and without brakes.


In this two situation, which is riskier, Hope fully your answer will be situation 2.

Because if the person driving a car with reduced brakes will increase the chances of accidents (Probability).

Understanding part 2:


Situation 1: Driving a bike with helmet.

Situation 2: Driving a bike without a helmet.

In these situations, which one is riskier, hopefully, your answer is situation 2.


Because, if the person is driving a bike without a helmet, if the accidents occur, the level of injury will be high (severity).

Understanding part 3:



Situation 1: The person driving a car with high speed.


Situation 2: The person driving a car with low speed.


Which one is riskier, you are right the risky is situation 1.


Because if the person is driving at high speed, it will increases both


Chances of accidents by loss of control (Probability) and


level of injury and damage if the accidents occur (severity)


The situation 1 increases both probability and severity.

Finally, the conclusion is that,



Probability = The chances for the accidents to occur or else the chances hazard to cause the harm

Severity = the resulting effect if occurs, or else the level of injury, damage, loss if the accident occurs.

Workplace example:

Some of the examples increasing the probability of accidents:
  • Un insulated or damaged insulation cables 
  • Erection of scaffold on unstable ground 
  • Use of crane without outriggers

Some of the examples increasing the severity of accidents:
  • High voltage power lines 
  • Work with carcinogen substance 
  • Working with X-ray without radiation protecting clothes. 

But again remember, the level of probability and severity cannot be determined by a single factor, it is important to consider the complete influencing factors such as


Influence of environment where the work is Carried out
Influencing personal factors such as competence, medical conditions, age, personal behaviour etc.
  • The nature of task 
  • The influences by weather conditions 
  • The nature of the substance or equipment’s used etc. 
So, it’s important to consider the entire influencing factors, before deciding the level of risk.


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