![]() The figure shows a record 1000 years of losses of different sizes (magnitudes) - nine events exceeded a loss of '60' over that period. What do we mean by return period? Take the following example adapted from the UNISDR Global Assessment Report 2015: In the context of disaster risk, probability refers to the frequency of occurrence or the return period of losses associated with hazardous events. This method assesses the likelihood of an event(s) and it contains the idea of uncertainty because it incorporates the concept of randomness. Probabilistic risk is the chance of something adverse occurring. In order to address this short-fall, we have to adopt a probabilistic approach. Consequently, deterministic scenario planning may actually be underestimate the potential risk. ![]() There are a number of problems with a deterministic approach, including the fact that it does not consider the full range of possible outcomes, and does not quantify the likelihood of each of these outcomes. running multiple scenarios at different probabilities of occurrence) can be used to generate a deterministic scenario typical scenarios might include: ![]() For example, probabilistic modelling (i.e. There is overlap in deterministic and probabilistic modelling. The deterministic approach typically models scenarios, where the input values are known and the outcome is observed. In contrast, a deterministic model treats the probability of an event as finite. Probabilistic models therefore "complete" historical records by reproducing the physics of the phenomena and recreating the intensity of a large number of synthetic events. As a result, these risk assessments resolve the problem posed by the limits of historical data. Probabilistic risk assessment simulates those future disasters which, based on scientific evidence, are likely to occur. While historical losses can explain the past, they do not necessarily provide a good guide to the future most disasters that could happen have not happened yet. OECD, 2012 What is the difference between deterministic and probabilistic risk? Probabilistic assessments are characterized by inherent uncertainties, partly related to the natural randomness of hazards, and partly because of our incomplete understanding and measurement of the hazards, exposure and vulnerability under consideration. Deterministic risk considers the impact of a single risk scenario, whereas probabilistic risk considers all possible scenarios, their likelihood and associated impacts.ĭeterministic approaches are used to assess disaster impacts of a given hazard scenario, whereas probabilistic methods are used to obtain more refined estimates of hazard frequencies and damages.
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