Civil Engineering Reference
In-Depth Information
There are a number of methods for evaluating risk.
The method applied for any particular risk will depend on
a number of factors, such as the complexity of the activ-
ities carried out and the type and nature of the workplace.
For many of the day-to-day risks that people in the work-
place are exposed to, including fi re, a simple qualitative
assessment will suffi ce, for more complex risks a quanti-
tative or semi-quantitative assessment may be needed.
As no numerical scales have been introduced with
the above simple estimator it is diffi cult for a precise
indication of the level of risk to be determined. Neither
does this approach provide an easy mechanism to
confi rm if the risk has been reduced to the lowest level
reasonably practicable.
However, by introducing the above subjective
estimations of the two elements of risk into a simple risk
matrix, a qualitative assessment of risk can be made.
Figure 5.10 is an example of a simple risk matrix.
When undertaking risk assessments for a number of
work activities the application of a consistent method of
qualitative evaluation of risk will allow for actions to be
prioritised.
For example, a systematic qualitative evaluation of
an organisation's risk will result in some risks that are
evaluated as 'high', some that are 'low' and some that are
'insignifi cant', management decisions can then be made
of a basis of risk vs cost. This qualitative method provides
a basic evaluation of risk and will allow an organisation to
consider what may or may not be considered a reason-
ably practical level of safety.
Qualitative analysis - describes the quality
of risk using words.
Quantitative analysis - quantifi es the risk
with numerical data.
Semi-quantitative analysis - uses numbers
to quantify qualitative data.
Qualitative analysis
Qualitative analysis describes a quality of the risk.
Typical of the qualities most often described is that of
quantum, i.e. size or magnitude. For example, when
assessing the means of escape in the event of a fi re, fi re
risk has historically been rated as high, normal or low.
Qualitative analysis of risk is a subjective measure,
based upon the risk assessor's judgement. As with all
methods of risk evaluation, a qualitative assessment will
need to allow consideration of the two aspects of risk,
i.e. the likelihood of a particular occurrence and the
severity of the consequences.
The HSE, in its guidance document HSG65, have
suggested a basic estimator as appropriate for a simple
evaluation of risk. The HSE estimator, shown in Table 5.1,
uses purely subjective measures of likelihood and uses
RIDDOR events to ascertain the qualitative descriptors
of the seriousness of the outcome.
Major
injuries may
occur
Serious
injuries may
occur
Slight
injuries may
occur
High
chance of an
event
High risk
Medium risk
Low risk
Medium
chance of an
event
Medium risk
Medium risk
Low risk
Low chance
of an event
Insignificant
risk
Low risk
Low risk
Likelihood of an accident/incident occurring
Outcome of the potential accident/incident
Figure 5.10
Example of a simple risk matrix using the
two aspects of risk to determine the magnitude of risk,
expressed in qualitative terms
Table 5.1
The HSE estimator
Likelihood
Severity
High
Where it is certain or near certain
Major
Death or major injury (as
that harm will occur
defi ned by RIDDOR) or
illness causing long-term
disability
Medium
Where harm will often occur
Serious
Injuries or ill health causing
short-term disability
Low
Where harm will seldom occur
Slight
All other injuries or illness
