Human error technique

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. 

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. 

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. 

ATHEA A Technique for Human Error Analysis NUREG/CR-6350... 

HEART Human Error rale Assessment and Reduction Technique Williams, 1988... 

THERP Technique for Human Error Rale Prediction NUREG/CR-I278, 198.)... 

S.2.9 Technique for Human Error Rate Prediction (THERP)... 

Chapter 5, Quantitative and Qualitative Prediction of Human Error in Safety Assessments, describes a systematic process for identifying and assessing the risks from human error, together with techniques for quantifying human error probabilities. 

Chapter 4 focuses on techniques which are applied to a new or existing system to optimize human performance or qualitatively predict errors. Chapter 5 shows how these teclmiques are applied to risk assessment, and also describes other techniques for the quantification of human error probabilities. Chapters 6 and 7 provide an overview of techniques for analyzing the underlying causes of incidents and accidents that have already occurred. 

The book provides a comprehensive set of examples and case studies that cover a wide variety of process plant situations. Some of these are intended to illustrate the range of situations where human error has occurred in the CPI (see Appendix 1). Other examples illustrate specific techniques (for example. Chapter 4 and Chapter 5). Chapter 7 contains a number of extended case studies intended to illustrate tedmiques in detail and to show how a range of different techniques may be brought to bear on a specific problem. 

An opportimity for error recovery would have been to implement a checking stage by a supervisor or independent worker, since this was a critical maintenance operation. However, this had not been done. Another aspect of the unforgiving environment was the vulnerability of the system to a single human error. The fact that the critical water jacket flow was dependent upon a single pump was a poor design that would have been detected if a hazard identification technique such as a hazard and operability study (HAZOP) had been used to assess the design. 

This chapter has reviewed various PIFs which deterirune the likelihood of human error in the CPI. The list of PIFs in Table 3.1 can be used by engineers and managers to evaluate and audit existing work systems, analyze process incidents and generate error reduction strategies in conjunction with the techniques described in Chapters 4 and 5. 

The various analytical methods for predicting and reducing human error can be assigned to four groups or sections. In order to make a start on any form of analysis or prediction of human error, it is obviously necessary to gather information. The first section therefore describes a number of techniques that can be applied to acquire data about what the worker does, or what happened in an accident. 

Task analysis is a fundamental methodology in the assessment and reduction of human error. A very wide variety of different task analysis methods exist, and it would be impracticable to describe all these techniques in this chapter. Instead, the intention is to describe representative methodologies applicable to different types of task. Techniques that have actually been applied in the CPI will be emphasized. An extended review of task analysis techniques is available in Kirwan and Ainsworth (1993). 

In addition to their descriptive fimctions, TA techniques provide a wide variety of information about the task that can be useful for error prediction and prevention. To this extent, there is a considerable overlap between Task Analysis and Human Error Analysis (HEA) techniques described later in this chapter. HEA methods generally take the result of TA as their starting point and examine what aspects of the task can contribute to human error, hr the context of human error reduction in the CPI, a combination of TA and HEA methods will be the most suitable form of analysis. 

The application of human error analysis (HEA) techniques is to predict possible errors that may occur in a task. The next stage of error analysis is to identify error recovery possibilities implicit within the task, and to specify possible... 

The other main application area for predictive error analysis is in chemical process quantitative risk assessment (CPQRA) as a means of identifying human errors with significant risk consequences. In most cases, the generation of error modes in CPQRA is a somewhat unsystematic process, since it only considers errors that involve the failure to perform some pre-specified function, usually in an emergency (e.g., responding to an alarm within a time interval). The fact that errors of commission can arise as a result of diagnostic failures, or that poor interface design or procedures can also induce errors is rarely considered as part of CPQRA. However, this may be due to the fact that HEA techniques are not widely known in the chemical industry. The application of error analysis in CPQRA will be discussed further in Chapter 5. 

Qualitative human error prediction is the most important aspect of assessing and reducing the human contribution to risk. For this reason, it will be described in some detail in this section. The qualitative analysis performed in SPEAR involves the following techniques ... 

If the results of the qualitative analysis are to be used as a starting-point for quantification, they need to be represented in an appropriate form. The form of representation can be a fault tree, as shown in Figure 5.2, or an event tree (see Bellamy et al., 1986). The event tree has traditionally been used to model simple tasks at the level of individual task steps, for example in the THERP (Technique for Human Error Rate Prediction) method for human reliability... 

THERP Techniques for human error rate prediction... 

History and Technical Basis. The influence diagram approach (IDA) (also known as the sociotechnical approach to human reliability (STAHR) (see Phillips et al., 1990) is a technique that is used to evaluate human error probabilities as a... 

Typically, the first phase of a comprehensive accident investigation process will involve describing the way in which the hardware, the chemical process, individual operators and operating teams are involved in the accident process. This is the domain of the structural analysis techniques and the technical analysis of the chemical process which gave rise to the accident. Analyses of human error will primarily address the interactions between hardware systems and individuals or operating teams (the first two layers... 

An extension of the tree of causes, called variation diagrams (Leplat and Rasmussen, 1984) was developed to answer some of these criticisms. In this method, the Rasmussen stepladder model of human error (see Chapter 2) is applied to analyze causal factors at each node of the tree. A detailed example of the use of this technique is provided in Chapter 7 (Case Study 1). 

This section illustrates how the techniques described in Chapter 4 can be used to develop a procedure for the job of the top floor operator in the batch plant considered earlier. Two techniques are illustrated (i) a hierarchical task analysis (HTA) of the job, and (ii) a predictive human error analysis (PHEA) of the operations involved. HTA provides a description of how the job is actually done while PHEA identifies critical errors which can have an impact on the system in terms of safety or quality. The basic structure of the procedure is derived from the HTA which specifies in increasing detail the goals to be achieved. To emphasize critical task steps, various warnings and cautions can be issued based on the likely errors and recovery points generated by the PHEA. 

Kirwan, B. (1992). Human Error Identification in Reliability Assessment. Part 2 Detailed comparison of techniques. Applied Ergonomics 23(6), 371-381. 

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