Civil Engineering Reference
In-Depth Information
Semi-quantitative analysis
The use of semi-quantitative techniques for risk assess-
ment is widespread and it is often referred to as a quan-
titative method; however, it is easily seen that risk is
expressed as a numerical value, the estimation of the
magnitude of the risk is in fact subjective and therefore
qualitative. A semi-quantitative evaluation of risk allows
numerical values to be assigned to both severity and
likelihood in the absence of data.
Like the qualitative approach, the semi-quantitative
technique is essentially subjective and is based upon the
risk assessor's personal interpretation of the level of risk,
but with the added assistance of a risk matrix so that a
risk factor can be calculated numerically.
For example, the likelihood of a particular event
occurring is assigned numerical values as shown in
Table 5.2.
While it is easy to differentiate between the two
opposing defi nitions at the end of each scale, there is
often confusion with the words used to separate out the
mid range.
There is no laid down criteria for either the words
used to describe the value or the numerical values within
the scale. A 5 by 5 scale is arbitrarily used here, though
there are a wide variety of scales used from 3 through to
8 and sometimes even 10.
Just as the likelihood rating is assigned numerical
values, so too is the severity rating. Again using a scale of
1 to 5 and adapting the HSE's simple qualitative estima-
tions of severity, an example of how numerical values
may be assigned is shown in Table 5.3.
In addition to the severity of the outcome of an
event for an individual, the total numbers of people that
may be affected must also be considered, as in the case
of calculating the likelihood of the rise in fi res started
by discarded smokers' materials in places of public
entertainment (if a smoking policy exists). The resultant
effect upon persons being able to escape safely may
also revolve around the numbers of persons present.
Having assigned numeric values to each element of
risk, a risk matrix can then be completed to provide a
calculation of an overall 'risk factor'.
The resultant risk matrix can be used to provide
and develop an action plan which may also be assigned
numbers so that priorities can be identifi ed. Risk
matrices are often colour coded to provide a visual
concept of whether or not the residual risk is tolerable or
acceptable.
An example of a semi-quantitative risk matrix, which
incorporates the numerical scales discussed above, is
shown in Figure 5.12.
Table 5.2
Example of numeric values given to subjective,
qualitative descriptions of risk, of a typical semi-quantitative
assessment
The qualitative description
Numerical value that
of the likelihood of a
may be assigned
particular event occurring
No evidence of occurrence
1
Foreseeable but remote
2
Has occurred but only infrequently
3
Has occurred fairly frequently
4
Has occurred regularly and will
5
occur again
Table 5.3
An example of how numerical values may be assigned
The qualitative description of the severity
Numerical value that
of a particular event
may be assigned
First aid injury or illness requiring minimal attention
1
(plaster, etc.)
Minor injury or illness. Includes those where a
2
person could spend
up to
3 days away from work
'3 day' injury or illness. Those where the person
3
would be off work more than 3 days (as per RIDDOR)
'Major' injury or illness (as defi ned per RIDDOR)
4
Fatality or disabling injury or illness preventing return
to work
5
Note:
When assigning numerical scales to severity the assessor must take into account chronic or long-
term effects of any particular event and/or the effects of the long-term exposure to a perceived 'low'
hazard.
