Chap6 Quantitative Risk Assessment

Question 1

Define Frequency

Answer 1

[Probability (a pure number) x Population Size] time period

Question 2

Define Probability

Answer 2

The chance of an event happening (measured on a scale from 0 - 1)

Question 3

Name the 3 Basic Concepts of Probability

Answer 3

1. ‘Classical’ concept
2. ‘Relative Frequency’ concept
3. ‘Subjective Probability’ concept

Question 4

What is the ‘Classical’ Concept?

Answer 4

Probability calculated based on symmetry & reason alone; e.g. the probability of a coin coming down heads

Question 5

What is the ‘Relative Frequency’ concept?

Answer 5

Probability is estimated as the number of events of concern divided by the total number of events. The probability concept that is used most in science & engineering in general & in risk assessment in particular.

Question 6

What is the ‘Subjective Probability’ concept

Answer 6

Probability relating to unique events & which reflect the point of view of the person making the estimate; the value would be specific to that person

Question 7

Name the 3 main ways of estimating probability

Answer 7

1. From Empirical Data
2. By Subjective Estimate
3. By Probabilistic Modelling

Question 8

Give an example of using Empirical Data to estimate a probability

Answer 8

Manufacturer making lightbulbs designed to last 1000 hours each. Tests from each batch show 1 in 400 fail to meet this. Hence probability light not lasting 1000 hours is 1/400=0.0025. Therefore probability of light lasting 100 hours = 1 - 0.0025 = 0.9975

Question 9

Give an example of using Subjective Estimate to estimate a probability

Answer 9

A specialist shop is run by a person who doesn’t open the shop during ‘normal’ work hours. A neighbour estimates that the shop is open 95% of the time during normal hours.

Question 10

When might a Subjective Estimate be used?

Answer 10

A Subjective Estimate may be used if physical reasoning is not possible & there is no data available.
NB: The likelihood of a hazard may be under or over-estimated reflecting a vested interest.

Question 11

Give an example of using Probabilistic Modelling

Answer 11

Calculating the probability that all the components of a system will work correctly when required - e.g. the sub-sea emergency shut down system on a oil rig

Question 12

What is meant by a ‘Frequentist Approach’?

Answer 12

Calculating frequencies used in risk assessments on the collection of test data, & more frequently, from field data of component or system failures & relevant experiences