Errors human

Tasks using skills and rules probably exceed those requiring a call on knowledge by about 1000 1. However, when we do call on knowledge, the possibility of error is usually greater. 

We tend to use similarity matching (which previous challenge does our current challenge most closely resemble ) or frequency gambling (which solution do we call up most often ). However these approaches can fail where the new challenge is sufficiently different from the ones we have previously responded to. 

Reason divided error into slips, or lapses, and mistakes. Slips or lapses involve the correct choice of a method to achieve a task, but a failure to carry it out correctly. Mistakes on the other hand involve a flaw in the plan which means that even if it is carried out correctly, the objective isn t achieved. Slips and lapses are often skill-and rale-based errors, whereas mistakes are often knowledge-based errors. 

Procedures need to be devised with the possibihty of error in mind, e.g. aity procedure which has a loop in it where the operator runs a check before moving on raises the possibility that the operator will re-enter the procedure at the next step regardless of the test 

Reason believed that after initially training people in a procedure, as error is almost certain to occur sooner or later, the next step is to train them to recover fiom anticipated errors. An understanding of human error and its various forms must form part of aity consideration of a safety management system, a procedure and an accident investigatioa 

Human failure can essentially be split into two categories circumstances in which our actions are unintentional and those which are deliberate. Inadvertent actions constitute human error whilst deliberate activities fall into the remit of violation. 

Cognitive psychologists describe different types of human performance [8]  

Human error is defined by James Reason as the failure of planned actions to achieve their desired ends - without the interveution of some unforeseeable event [1]. Depending on the mode in which we are performing, different types of errors might occur. These are often referred to as slips, lapses and mistakes. 

Lapses are similar to slips but pertain to situations where we omit an action or forget what our original intention was. Lapses are therefore related to memory, examples might include  

Slips and lapses often occur when the task being undertaken is overly complex or long-winded. Similarly we may be susceptible when we are dealing with steps 

In the preceding sections only failures of technical components were treated. This is not enough for the analysis of a technical system. Building, operating and maintaining a technical plant requires human interventions. The extent to which these are necessary depends on the degree of automation of the plant. In general 

Human error is defined as an act outside the tolerance bounds. These are determined by the technical boundary conditions and may therefore be influenced— within limits— by the designer in the sense that the tolerance region becomes large (fault-tolerant design). This reduces the probability of human error. 

Before dealing with modelling human error, a classification of errors is useful. This can be done in many ways. A universally accepted classification does not exist. In what follows the classifications of [52] are presented. Accordingly two broad categories of human error may be distinguished  

Systems analysis usually only deals with human error due to the work environment. The following classification can be found for this. It is based on the possibilities for human error derived from the ways of human information processing [52]. 

The actions mentioned above are composed of one or several tasks or steps. Intentional errors such as sabotage are normally not addressed, since their probability can virtually not be determined. 

A total of 21 railroad cars carrying caustic soda, chlorine, propane, styrene, and toluene derailed. Three of the railroad cars carrying propane and toluene exploded and caught fire while a fourth railroad car carrying chlorine ruptured and its contents also caught fire. No lives were lost, but eight fire fighters were injured and 250,000 people evacuated. 

The escape of some 40 tons of MIC (methyl isocyanate) gas from a Union Carbide pesticide production plant in the Indian city of Bhopal led to the world s worst industrial disaster. At least 2,500 people were killed, 10,000 seriously injured, 20,000 partially disabled, and 180,000 others adversely affected in one way or another. Some 150,000 people are reported to be still suffering from the adverse effects of the Bhopal catastrophe. 

Human error can be defined as an action that is inconsistent with established behavioral patterns (speeding ticket, public intoxication, etc.) considered normal or that differs from prescribed procedures. Errors can be divided into two categories predictable and random. Predictable errors are those which experience has shown will occur again if the same conditions exist. Predictable errors can be foreseen because their occurrence has taken place more 

In the workplace errors are further sorted into two types, errors of omission and errors of commission. 

Some following paragraphs discuss the various types of human errors. 

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This work allows the optimization of the testing conditions in order to minimize human error in the inteipretation and obtaining of an reproducible and clear anomaly spectrum. 

Process Hazards Analysis. Analysis of processes for unrecogni2ed or inadequately controUed ha2ards (see Hazard analysis and risk assessment) is required by OSHA (36). The principal methods of analysis, in an approximate ascending order of intensity, are what-if checklist failure modes and effects ha2ard and operabiHty (HAZOP) and fault-tree analysis. Other complementary methods include human error prediction and cost/benefit analysis. The HAZOP method is the most popular as of 1995 because it can be used to identify ha2ards, pinpoint their causes and consequences, and disclose the need for protective systems. Fault-tree analysis is the method to be used if a quantitative evaluation of operational safety is needed to justify the implementation of process improvements. 

Chemical Reactivity Evaluation and Application to Process Design Preventing Human Error in Process Safety... 

Monitoring by Electromechanical Instrumentation. According to basic engineering principles, no process can be conducted safely and effectively unless instantaneous information is available about its conditions. AH sterilizers are equipped with gauges, sensors (qv), and timers for the measurement of the various critical process parameters. More and more sterilizers are equipped with computerized control to eliminate the possibiUty of human error. However, electromechanical instmmentation is subject to random breakdowns or drifts from caUbrated settings and requires regular preventive maintenance procedures. 

ETA breaks down an accident iato its contributing equipment failures and human errors (70). The method therefore is a reverse-thinking technique, ie, the analyst begias with an accident or undesirable event that is to be avoided and identifies the immediate cause of that event. Each of the immediate causes is examined ia turn until the analyst has identified the basic causes of each event. The fault tree is a diagram that displays the logical iaterrelationships between these basic causes and the accident. 

The result of the ETA is a Hst of combiaations of equipment and human failures that ate sufficient to result ia the accident (71). These combiaations of failures are known as minimal cut sets. Each minimal cut set is the smallest set of equipment and human failures that are sufficient to cause the accident if all the failures ia that minimal set exist simultaneously. Thus a minimal cut set is logically equivalent to the undesired accident stated ia terms of equipment failures and human errors. 

Minimal cut sets are then ranked. Two factors are considered in the ranking procedure. The first factor considers stmcture, ie, a one-event minimal cut set is more important than a two-event minimal cut set. The implication is that one event is more likely to occur than two events, two events are more likely than three events, and so on. The second factor considers ranking within equal-size minimal cut sets. The general ranking rules consider the probabihty of human error, active equipment failure, and passive equipment failure (73). 

This ranking implies that human errors are more likely to occur than active equipment failures (functioning equipment, such as a mnning pump) and that active equipment failures are more likely to occur than passive equipment failures (static, nonfunctioning equipment, such as a storage tank). 

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. 

Another difficulty is assessing the potential for human errors. If redundancy is accompanied with increased complexity, the resulting increased potential for human errors must be taken into consideration. Redundant systems require maintenance procedures that can correct problems in one part of the system while the remainder of the system is in full operation. When conducting maintenance in such situations, the consequences of human errors can be rather unpleasant. 

The possibility of a human error in the maintenance of the process controls having consequences for the safety interlock system is eliminated. 

Inherently Safer Design Rather than add on equipment to control hazards or to protect people from their consequences, it is better to design user-friendly plants which can withstand human error and equipment failure without serious effects on safety, the environment, output, and efficiency. This part is concerned with this matter. 

For many years the usual procedure in plant design was to identify the hazards, by one of the systematic techniques described later or by waiting until an accident occurred, and then add on protec tive equipment to control future accidents or protect people from their consequences. This protective equipment is often complex and expensive and requires regular testing and maintenance. It often interferes with the smooth operation of the plant and is sometimes bypassed. Gradually the industry came to resize that, whenever possible, one should design user-friendly plants which can withstand human error and equipment failure without serious effects on safety (and output and emciency). When we handle flammable, explosive, toxic, or corrosive materials we can tolerate only very low failure rates, of people and equipment—rates which it may be impossible or impracticable to achieve consistently for long periods of time. 

At one time most accidents were said to be due to human error, and in a sense they all are. If someone—designer, manager, operator, or maintenance worker—had done something differently, the accident would not have occurred. However, to see how managers and supervisors can prevent them, we have to look more closely at what is meant by human error-. 

Human error probabilities can also be estimated using methodologies and techniques originally developed in the nuclear industry. A number of different models are available (Swain, Comparative Evaluation of Methods for Human Reliability Analysis, GRS Project RS 688, 1988). This estimation process should be done with great care, as many factors can affect the reliability of the estimates. Methodologies using expert opinion to obtain failure rate and probability estimates have also been used where there is sparse or inappropriate data. 

Manual mode control operation is very common leading to increased potential for human error. 

Operator has to observe the process more frequently as a result of nonsteady state operating conditions. This requires more frequent control interventions, leading to an increased potential for human error. 

In general, the risk of human error ean he redueed hy properly designing the equipment, proeedures, and the work environment and hy proper staffing, training, and implementation of management eontrols. 

Proper design of equipment, procedures and the work environment can greatly reduce the probability of human error. Designing and maintaining operating procedures is a challenge for batch systems because of the multiplicity of procedures for each piece of equipment, and the variety of operations within... 

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. 

A number of error classification schemes have been developed over the years. These schemes can help provide a systematic framework for looking at error. Two schemes for classification of human errors are outlined in this section. 

Another common approach is to use an information-processing model to classify human errors. The classification models the information processing which occurs when a person operates and controls complex systems such as processing plants. One such classification (Rouse and Rouse, 1983) identifies six steps in information processing. Exhibit 6.1 lists the six steps, and provides some examples of errors that can occur at each of these steps. 

Applying the information-processing model to each of the operator tasks can provide insights into the potential for human error and also suggest solutions for preventing errors. 

Exhibit 6.2. Human Errors in Continuous and Batch Processes... 

The potential for a wide range of human errors is greater in batch processes than in continuous processes. Some examples can be found in the categories of errors in Exhibit 6.2. 

Safety issues in batch reaction systems relating to human errors and procedures are presented in Table 6. The table is meant to be illusttative but not comprehensive. 

Listed below are operator related safety issues that are more prevelant in batch operations. Keep in mind, however, that human error consists of more facets than operator error alone. 

Rasmussen, J. 1979. Notes on human error analysis and prediction. In G. Apostalakis and G. Volta (Eds.), Synthesis and Analysis Methods for Safety and Reliability Studies, Plenum, New York. 

Rouse, W. and S. Rouse 1983. Analysis and classification of human error. IEEE transactions on Systems, Man, and Cybernetics, SMC-13(4), 539-549. 

Sucb a construction is cumbersome and requires utmost caution to ensure that the terminals are properly disengaged before the trolley is racked-out. Otherwise it may pull the wires and snap the connections and result in a major repair. It is also possible that, due to human error, the operator may slip to engage the terminals at the first attempt and may have to do it at a second attempt, adding to the downtime, while energizing or replacing a faulty trolley, eventually defeating the purpose of a draw-out system. 

Checking for any inadvertent human error during assembly. 

Automatic correction is always recommended to eliminate manual dependence and to achieve better accuracy. It also elimintites the risk of a leading power factor by a human error that may cause an excessive voltage at the motor and the control gear terminals. 

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