196
13 Tolerance and Acceptability
Zone 3
This zone is the result of a trend to overestimate frequency and outcomes (Kumamoto
and Henley 2000), resulting in risk aversion for outcomes possibly including fatalities. A reasonable explanation of this phenomenon was given (Bohnenblust and
Schneider 1987) in Switzerland. According to them, poor estimations of risks after
severe accidents are one of the major reasons for risk-aversive attitudes, which prefer ten single-fatality accidents to one accident with ten fatalities. Thus, risks can
be mis-estimated or overestimated with respect to size or likelihood of outcomes,
generally resulting in Zone 3.
Zone 1
This area corresponds to high-probability low-consequence scenarios, as for example
having a piece of equipment which breaks down often, but does not really have any
significant consequence: despite the fact that the risks are lower than the constant
risk curve, clients will often decide to change the equipment, because the impact is
overestimated and/or by a “fatigue” process: this is often voiced as “that piece of
equipment was a pain in the neck, so we changed it!”.
Zone 2
This is the area where we find ourselves in a “false comfort zone” where we tend
to play with likelihood and consequences, often overestimating our tolerance: when
consequences are neither too heavy nor too frequent, we do love to take risks, and
this indeed is one of the engines of evolution.
To conclude, it is precisely in defining the tolerance curve in Zone 2 that the most
care has to be exerted by the risk assessments team, especially when considering the
influence of PAFs (Table 13.3).
References
[AGSLT 2007] Australian Geomechanics Society Landslide Taskforce (2007) Practice Note Guidelines For Landslide Risk Management, Landslide Practice Note Working Group, Journal and News
of the Australian Geomechanics Society
[APEGBC 2014] APEGBC 2014. Association of Professional Engineers and Geoscientists of
British Columbia (2014) Professional Practice Guidelines – Legislated. Dam Safety Reviews
in BC V2.0. https://www.apeg.bc.ca/For-Members/Professional-Practice/Professional-PracticeGuidelines
[ANCOLD 2003] Australian National Committee on Large Dams (2003) Guidelines on risk assessment. Sydney: ANCOLD
Bohnenblust H, Schneider T (1987) Risk appraisal: Can it be improved by formal decision models?
In: Uncertainty in Risk Assessment, Risk Management, and Decision Making, ed. VT Covello,
et al., pp. 71–87, New York, Plenum Press
Comar C (1987) Risk: A pragmatic de minimis approach, in: De Minimis Risk, ed C. Whipple, pp.
xiii–xiv, New York, Plenum Press
Christou M, Struckl, M, Biermann, T (2006) Land use planning guidelines in the context of article 12
of the Seveso II Directive 96/82/EC as amended by Directive105/2003/EC. European Commission
Joint Research Centre, Institute for the Protection and Security of the Citizen. EUR 22634 EN
FR DE
13 Tolerance and Acceptability
Zone 3
This zone is the result of a trend to overestimate frequency and outcomes (Kumamoto
and Henley 2000), resulting in risk aversion for outcomes possibly including fatalities. A reasonable explanation of this phenomenon was given (Bohnenblust and
Schneider 1987) in Switzerland. According to them, poor estimations of risks after
severe accidents are one of the major reasons for risk-aversive attitudes, which prefer ten single-fatality accidents to one accident with ten fatalities. Thus, risks can
be mis-estimated or overestimated with respect to size or likelihood of outcomes,
generally resulting in Zone 3.
Zone 1
This area corresponds to high-probability low-consequence scenarios, as for example
having a piece of equipment which breaks down often, but does not really have any
significant consequence: despite the fact that the risks are lower than the constant
risk curve, clients will often decide to change the equipment, because the impact is
overestimated and/or by a “fatigue” process: this is often voiced as “that piece of
equipment was a pain in the neck, so we changed it!”.
Zone 2
This is the area where we find ourselves in a “false comfort zone” where we tend
to play with likelihood and consequences, often overestimating our tolerance: when
consequences are neither too heavy nor too frequent, we do love to take risks, and
this indeed is one of the engines of evolution.
To conclude, it is precisely in defining the tolerance curve in Zone 2 that the most
care has to be exerted by the risk assessments team, especially when considering the
influence of PAFs (Table 13.3).
References
[AGSLT 2007] Australian Geomechanics Society Landslide Taskforce (2007) Practice Note Guidelines For Landslide Risk Management, Landslide Practice Note Working Group, Journal and News
of the Australian Geomechanics Society
[APEGBC 2014] APEGBC 2014. Association of Professional Engineers and Geoscientists of
British Columbia (2014) Professional Practice Guidelines – Legislated. Dam Safety Reviews
in BC V2.0. https://www.apeg.bc.ca/For-Members/Professional-Practice/Professional-PracticeGuidelines
[ANCOLD 2003] Australian National Committee on Large Dams (2003) Guidelines on risk assessment. Sydney: ANCOLD
Bohnenblust H, Schneider T (1987) Risk appraisal: Can it be improved by formal decision models?
In: Uncertainty in Risk Assessment, Risk Management, and Decision Making, ed. VT Covello,
et al., pp. 71–87, New York, Plenum Press
Comar C (1987) Risk: A pragmatic de minimis approach, in: De Minimis Risk, ed C. Whipple, pp.
xiii–xiv, New York, Plenum Press
Christou M, Struckl, M, Biermann, T (2006) Land use planning guidelines in the context of article 12
of the Seveso II Directive 96/82/EC as amended by Directive105/2003/EC. European Commission
Joint Research Centre, Institute for the Protection and Security of the Citizen. EUR 22634 EN
FR DE