Low probability events

Note that some of the risk information is actuarial (based on statistical data, typically collected and organized by insurance companies), and some of it has been derived from the type of risk assessment discussed in this book (chloroform in chlorinated drinking water, afla-toxin in peanut products). While the uncertainties associated with the figures in Table 11.2 are much greater for some risks than for others (not a trivial problem in presentation of risk data), such a presentation, it would seem, is helpful to people who are trying to acquire some understanding of extremely low probability events, of the order of one-in-one million. 

One of Wilson s more interesting presentations is that depicted in Table 11.3. Here exposures or activities associated with annual (not lifetime) risks of one in 1 000 000 have been described, another useful way to help people gain some sense of the reality of very low probability events. 

In The Blind Watchmaker Richard Dawkins tells his readers that even if a statue of the Virgin Mary waved to them, they should not conclude they had witnessed a miracle.9 Perhaps all the atoms of the statue s arm just happened to move in the same direction at once—a low-probability event to be sure, but possible. Most people who saw a statue come to life would... 

A third example was the presence of cocaine in an individual s urine due to accidental ingestion. Inca tea, a tea made from purportedly decocainized cocaine leaves purchased in a health food store, was brewed and ingested by an individual who was later found to be positive for cocaine. If the cocaine had been removed from the leaves as stated, the product would have been perfectly legal and could not have contributed to a positive urinalysis (low probability event number one). Likewise, if the individual had not been tested within a few hours of consuming the tea, the results probably would have been negative (low probability event number two). In this case, a sample of the tea bag was produced, analyzed, and found to contain cocaine in sufficient quantities to cause a positive urinalysis (low probabihty event number three). Further investigation revealed that Inca tea leaves were ground and dried cocaine leaves and not decocainized. The product was immediately removed from the market. - ... 

The most frequently cited low probability event, that became apparent as a defense argument for urinalysis, was passive exposure to marijuana smoke. An explanation for a marijuana positive urinalysis was that the service member was exposed to significant quantities of marijuana sidestream smoke in close proximity to the service member. Because this defense may have had validity, the Navy commissioned subsequent scientific studies to resolve this issue. These studies showed conclusively that environmental exposure would result in a marijuana positive, providing the exposure was severe enough. However, in the experiment the exposure needed to be repeated for many hours and at a level such that the subjects needed to wear eye goggles to protect themselves from the marijuana smoke. Exposures at these levels could hardly be considered to be inadvertent or unwilling. As a result of these studies, cutoff levels could be defended and the credibUity of the passive exposure defense was diminished. 

Based on systematization of information on the decommissioning and environmental rehabilitation objects, a list of the most significant accidents was drawn up for each type of objects. Accidents with insignificant consequences (for example, spillage of LRW) and low probability events were not considered at all. 

ATP formed during reaction (7), thereby making the formation of radioactive ADP a low probability event. 

Despite the fact that planning is usually considered the most important step in disaster preparedness and response, it is frequently the step that receives the least attention. Auf der Heide (1989) attributes this to the apathy factor. He reports that because disasters are low-probability events, disaster preparedness tends to... 

The assembly of multimeric enzymes requires very specific orientation of two or more partially folded subunits. The encounter and precise interaction of two large and complex macromolecules in solution is normally a low probability event. As a result, in-vitro in the absence of additional factors, the correct assembly of protein subunits... 

A particular kind of electronic relaxation process is electron transfer. In this case (see Chapter 16) the electronic transition is associated with a large rearrangement of the charge distribution and consequently a pronounced change of the nuclear configuration, which translate into a large A. Nuclear tunneling in this case is a very low-probability event and room temperature electron transfer is usually treated as an activated process. 

The need for more refined tests, a better understanding of what is going on during an impact in an explosive, and Improve statistical approaches to the treatment of low-probability events are the paramount requirements for future studies in impact sensitivity. 

Numerous problems remain to be solved before it will be possible to quantify the sensitivity of explosives to impact and friction in terms that express an intrinsic property of the chemical substances. The need for more refined tests, a better understanding of the phenomena that occur during the test procedures, and improved statistical approaches to the treatment of low-probability events are paramount requirements for more meaningful data. Even then it will continue to be difficult to relate measurements in laboratory apparatus to the conditions encountered in the industrial handling and field utilization of azides. 

On the other hand, the filtering process of bounded rationality can lead to workers overweighting low probability events. This paradoxical result follows from the fact that the few such events not discarded will stand out in a reduced field of vision. Suppose, for example, that workers pay attention to the slim but catastrophic possibility of contracting lung cancer from airborne benzene. If as a result of bounded rationality this is one of only a half dozen or so job characteristics they register, they are likely to attach even more importance to this one risk than it deserves. ... 

Camerer, Colin F. and Howard Kunreuther. 1989. Decision Processes for Low Probability Events Policy Implications. Journal of Policy Analysis and Management. 8(4) 565-92. 

Most accidents in well-designed systems involve two or more low-probability events occurring in the worst possible combination. When people attempt to predict system risk, they explicitly or implicitly multiply events with low probability— assuming independence—and come out with impossibly small numbers, when, in fact, the events are dependent. This dependence may be related to common systemic factors that do not appear in an event chain. Machol calls this phenomenon the Titanic coincidence [131]. ... 

Alarms are used to alert controllers to events or conditions in the process that they might not otherwise notice. They are particularly important for low-probability events. The overuse of alarms, however, can lead to management by exception, overload and the incredulity response. 

Pradlwarter, H. J. and Schueller, G. I. Assessment of low probability events of dynamical systems by controlled Monte Carlo simulation. Probabilistic Engineering Mechanics (1999), 213-227. 

The causes of major events are typically complex and involve a series of low-probability events. It is very difficult to write standards and regulations to cover all such complex trains of events. 

If risk-scoring systems are apphed rigidly whereby a higher score indicates a greater risk than a lower score, and the scores are not considered in light of the employee, conununity, social, and financial concerns of the organization, its best interests may not be weU served. This applies especially to low probability events that may have severe consequences, for which risk scores may be low [p. 171]. 

Williams, M.M.R. and M.C. Thome (1997). Estimation of failure rates for low probability events. Progress in Nuclear Energy 31, 373-476. 

Issue 2 was considered by a joint industry-AECB ad-hoc technical working group, which proposed public risk criteria appropriate to small reactors AECL identified the low-probability events for Issue 3, although little debate occurred on this. Issue 4 and part of 7 were made obsolete when the liquid poison shutdown system was replaced by the passive devices in the fuel elements. Passive self-starting hydrogen recombiners were used in the cover gas space. For issue 6, a commitment was made to perform a risk assessment, as is done on other AECL small reactors and on CANDUs. 

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