Lecture 3 - Social and individual risks

Question 1

What are some risk specific concepts?

Answer 1

• Estimates of Societal risk
• Societal risk estimation
• Frequency versus number of fatalities (F-N) curve
• Estimates Potential loss of life (PLL)
• Risk to groups (not individuals) in a specific situation (location or time period)
• Presentation of actual data: Fatal accident rate (FAR) and Fatality Rate

Question 2

What is an example of societal risks?

Answer 2

Societal risk is the risk of defined consequences related to the population that may be affected by hazardous events in a certain region. Refers to the realisation of a risk where many are likely to be negatively affected.
Calculated as number of events/time x consequences/ event
EX: chemical plants where liquefied toxic substances are manufactured, stored,
or used

Question 3

What is an example of individual risk?

Answer 3

Individual risk concerns the annual probability of a negative event on a single individual.
Calculated as societal risk / population at risk

Question 4

What is an F-N curve fro representing societal risks?7

Answer 4

REFER TO SLDIES
- An F-N curve can be used to show historical data for different hazards at a societal level.
- F-N curves display the probability of having N or more fatalities per year, as a function of N, on a double logarithmic scale.
- Y-axis – fatality frequency at which N or more people are affected.
- For a number of fatalities N from event i, the frequency 𝐹𝑁= ∑𝑖 𝐹𝑖
- 𝐹𝑁 is the cumulative frequency of all events i where the number of fatalities is greater or equal to N

Question 5

Expectation value (EV) or Potential loss of life (PLL)?

Answer 5

• EV is the average number of persons receiving the specified level of harm per year
• For fatalities this is called PLL which is a measure of how many fatalities per year are implied in the risk assessment of a situation.
• The Expectation Value (EV) or Potential Loss of Life (PLL) is simply the sum of the products of all the f-N pairs, i.e. Potential Loss of Life = ΣfN [people/year]
• The EV or PLL, sometimes also called the average Rate Of Death (ROD), is useful in Cost Benefit Analysis and ALARP demonstrations as it can be multiplied by the Value of a Statistical Life (e.g. ~ £1 million [HSE ALARP Suite of guidance]) to determine the value of the benefit that could be gained by reducing the risk.
• Mathematically, EV=ΣfN.
• Whilst the Expectation Value is reported with units “expected casualties per year”, the value in real cases is typically very small (e.g. 0.000 01 expected casualties per year).

Question 6

How to report actual data e.g. Fatal Accident Rate (FAR)?

Answer 6

REFER TO SLDIES41