Likelihood, definition

As microprocessor-based controls displaced hardwired electronic and pneumatic controls, the impac t on plant safety has definitely been positive. When automated procedures replace manual procedures for routine operations, the probability of human errors leading to hazardous situations is lowered. The enhanced capability for presenting information to the process operators in a timely manner and in the most meaningful form increases the operator s awareness of the current conditions in the process. Process operators are expected to exercise due diligence in the supervision of the process, and timely recognition of an abnormal situation reduces the likelihood that the situation will progress to the hazardous state. Figure 8-88 depicts the layers of safety protection in a typical chemical jdant. 

To be able to systematically identify opportunities for reducing human error, it is useful to ask the question, What is human error One definition is that human error is an inappropriate or undesirable human decision or behavior that reduces, or has the potential for reducing safety or system performance (Rasmusssen 1979). There is a tendency to view errors as operator errors. However, the error may result from inadequate management, design, or maintenance of the system. This broader view which encompasses the whole system can help provide opportunities for instituting measures to reduce the likelihood of errors. 

The expression for the likelihood of occurrence of an event or an event sequence during an interval of time or the likelihood of the success or failure of an event on test or on demand. By definition, probability must be expressed as a number ranging from 0 to 1... 

From a reliability engineering perspective, error can be defined by analogy with hardware reliability as "The likelihood that the human fails to provide a required system function when called upon to provide that fimction, within a required time period" (Meister, 1966). This definition does not contain any references to why the error occurred, but instead focuses on the consequences of the error for the system (loss or unavailability of a required function). The disadvantage of such a definition is that it fails to consider the wide range of other actions that the human might make, which may have other safety implications for the system, as well as not achieving the required function. 

There are many definitions of the word risk. It is a combination of uncertainty and damage a ratio of Itazards to safeguards a triplet combination of event, probability, and consequences or even a measure of economic loss or human injury in terms of both the incident likelihood and tlie magnitude of the loss or injuiy (AICliE, 1989). People face all kinds of risks eveiyday, some voluntarily and otliers involuntarily. Tlierefore, risk plays a very important role in today s world. Studies on cancer caused a turning point in tlie world of risk because it opened tlie eyes of risk scientists and healtli professionals to tlie world of risk assessments. 

It follows from the definition cited that the size of the zeta potential depends on the structure of the diffuse part of the ionic EDL. At the outer limit of the Helmholtz layer (at X = X2) the potential is j/2, in the notation adopted in Chapter 10. Beyond this point the potential asymptotically approaches zero with increasing distance from the surface. The slip plane in all likelihood is somewhat farther away from the electrode than the outer Helmholtz layer. Hence, the valne of agrees in sign with the value of /2 but is somewhat lower in absolute value. 

Qualitative frequency evaluation involves defining broad categories of event frequency, which can be used to assess the likelihood of occurrence of a specific incident outcome (consequence). These categories cover a full spectmm of frequencies, from those representing events that are likely to those that are highly unlikely. Definitions of likelihood categories vary, but Table 5.4 presents a typical list and definitions. 

The basis upon which this concept rests is the very fact that not all the data follows the same equation. Another way to express this is to note that an equation describes a line (or more generally, a plane or hyperplane if more than two dimensions are involved. In fact, anywhere in this discussion, when we talk about a calibration line, you should mentally add the phrase ... or plane, or hyperplane... ). Thus any point that fits the equation will fall exactly on the line. On the other hand, since the data points themselves do not fall on the line (recall that, by definition, the line is generated by applying some sort of [at this point undefined] averaging process), any given data point will not fall on the line described by the equation. The difference between these two points, the one on the line described by the equation and the one described by the data, is the error in the estimate of that data point by the equation. For each of the data points there is a corresponding point described by the equation, and therefore a corresponding error. The least square principle states that the sum of the squares of all these errors should have a minimum value and as we stated above, this will also provide the maximum likelihood equation. 

Systemic adjuvant therapy is the administration of systemic therapy following definitive local therapy (surgery, radiation, or both) when there is no evidence of metastatic disease but a high likelihood of disease recurrence. The goal of such therapy is cure. 

The number of Sis, present in today s chemical process industry is overwhelming as discussed by Tixier (Tixier et al., 2002). These indicators are categorized in several ways in literature, for example pro-active versus reactive indicators. Many of these categories are not unambiguous. Some authors, like Kletz (Kletz, 1998) define proactive as prior to the operational phase of an installation while other authors, like Rasmussen et al. (Rasmussen et al., 2000), define pro-active as prior to an accident. In this thesis two categories of indicators are used, i.e. pro-active and reactive indicators. Here the definition of Rasmussen (Rasmussen et al., 2000) is adopted, who defined pro-active indicators as indicators before an accident and reactive indicators as indicators after an accident. Moreover, the pro-active indicators are divided into predictive and monitoring indicators. The monitoring indicators use actual events as a measure for the likelihood, while the predictive indicators predict the likelihood. 

In the previous Chapter it was shown that most accidents are preceded by deviations in the operational process, e.g. Heinrich (Heinrich, 1959), Turner (Turner, 1978), Leplat (Leplat, 1987), Reason (Reason, 1997), etc. Additionally, it was shown that a specific class of deviations is present which is not covered by current pro-active safety indicators. These deviations are characterised by a high likelihood and low perceived safety related consequences and were defined as precursors and re-occur in the operational process of the organization prior to an accident. In order to find these deviations in a real life operation and to eventually find their underlying causes, the concepts of re-occurring deviation and operational process have to be explained in more detail. The various definitions and concepts derived in this Chapter are necessary to understand the next Chapters, which shows how they are applied in practice. 

Accordingly, isotopic equilibration for Cr and Se species is expected to be much slower than for the aqueous Fe(III)-Fe(II) couple, which reaches equilibrium within minutes in laboratory experiments (Beard and Johnson 2004). Additionally, Cr(III) and Se(0) are highly insoluble and their residence times in solution are small, which further decreases the likelihood of isotopic equilibration. In the synthesis below, isotopic fractionations are assumed to be kinetically controlled unless otherwise stated. However, definitive assessments of this assumption have not been done, and future studies may find that equilibrium fractionation is attained for some reactions or rmder certain conditions. 

Antidepressants have been shown effective in the treatment of major depression with response rates at approximately 60-70%. The only treatment for depression consistently shown to be more effective is ECT with response rates of 80-90%. There is no definitive means of predicting which medication will work best for a given patient nevertheless, the choice of a medication should not be made capriciously. Several factors can guide medication selection and thereby maximize the likelihood of a successful response. 

Historically, the treatment of alcohol use disorders with medication has focused on the management of withdrawal from the alcohol. In recent years, medication has also been used in an attempt to prevent relapse in alcohol-dependent patients. The treatment of alcohol withdrawal, known as detoxification, by definition uses replacement medications that, like alcohol, act on the GABA receptor. These medications (i.e., barbiturates and benzodiazepines) are cross-tolerant with alcohol and therefore are useful for detoxification. By contrast, a wide variety of theoretical approaches have been used to reduce the likelihood of relapse. This includes aversion therapy and anticraving therapies using reward substitutes and interference approaches. Finally, medications to treat comorbid psychiatric illness, in particular, depression, have also been used in attempts to reduce the likelihood of relapse. 

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