THERP analysis

A THERP analysis considers different types of error, such as not following an instruction, choosing a wrong switch or skipping a step in a sequence of activities, and forecasts the error rate for each of these tasks. If a person can make more than one type of error when carrying out a task, then the probabilities are added to one another. For example, when opening a valve an operator may ... 

A THERP analysis is most effective when the tasks are routine and when there is little stress. 

In effect, workers are treated as being components in a system, just like equipment items. Hence the THERP analysis can be integrated into probabilistic risk assessment (PRA) analyses—particularly fault and event trees—topics that are discussed in depth in Process Risk and Reliability Management. A THERP analysis is most effective when the tasks are routine and proceduralized, and when the persons involved are not under stress. 

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... 

PROBLEM DEFINITION. This is achieved through plant visits and discussions with risk analysts. In the usual application of THERP, the scenarios of interest are defined by the hardware orientated risk analyst, who would specify critical tasks (such as performing emergency actions) in scenarios such as major fires or gas releases. Thus, the analysis is usually driven by the needs of the hardware assessment to consider specific human errors in predefined, potentially high-risk scenarios. This is in contrast to the qualitative error prediction methodology described in Section 5.5, where all interactions by the operator with critical systems are considered from the point of view of their risk potential. 

QUALITATIVE ERROR PREDICTION. The first Stage of quantitative prediction is a task analysis. THERP is usually applied at the level of specific tasks and the steps within these tasks. The form of task analysis used therefore focuses on the operations which would be the lowest level of a hierarchical task analysis... 

This approach is illustrated by the development of event trees and fault tree analysis. In fault tree analysis, the probability of an accident is estimated by considering the probabihty of human errors, component failures, and other events. This approach has been extensively applied in the field of risk analysis (Gertman and Blackman 1994). THERP (Swain and Guttman 1983) extends the conditioning approach to the evaluation of human reliability in complex systems. 

The technique for human error-rate prediction (THERP) [ Swain and Guttmann, 1980] is a widely applied human reliability method (Meister, 1984] used to predict human error rates (i.e., probabilities) and the consequences of human errors. The method relies on conducting a task analysis. Estimates of the likelihood of human errors and the likelihood that errors will be undetected are assigned to tasks from available human performance databases and expert judgments. The consequences of uncorrected errors are estimated from models of the system. An event tree is used to track and assign conditional probabilities of error throughout a sequence of activities. 

ABSTRACT In Human Reliability Analysis (HRA) the assessment of dependence between human failure events refers to evaluating the influence of the failure on one task on the performance of the subsequent task. In Probabilistic Safety Assessments (PSAs), human action dependencies are commonly evaluated with the THERP method, often extended with Decision Tree (DT) models, to reduce the expert judgment element. This paper compares different DT models used in the HRA practice. The comparison addresses the factors entering the models and the underlying relationships. The comparison shows that, depending on the features of the task under analysis, the results may vary substantially if different DTs are used. Also, often there is limited guidance for the analyst in the assessment of the DT factors this prejudices the repeatability of the assessments because different analysts may very well decide for different assessments. 

The user of the method must analyze the pair of successive tasks and assess the level of dependence. To support the analysis of the tasks dependence, the THERP guidelines suggest the factors that should be considered (THERP Table 10-1 ofSwain Gullman(1983)) closeness in lime and space, functional relatedness (e.g. tasks related to the same subsystem), stress, similarity of the performers (status, training, responsibihty, and many social and psychological factors ). 

The hazard identification and evaluation of a complex process by means of a diagram or model that provides a comprehensive, overall view of the process, including its principal elements and the ways in which they are interrelated. There are four principal methods of analysis failure mode and effect, fault tree, THERP, and cost-benefit analysis. Each has a number of variations, and more than one may be combined in a single analysis. See also Cost-Benefit Analysis Failure Mode and Effects Analysis (FMEA/FMECA) Fault Tree Analysis (FTA) THERP (Technique for Human Error Rate Probability). 

THERP is usually modeled using a probability tree. Each branch represents a task analysis showing the flow of task behaviors and other associations. A probability is assigned based on the event s occurrence or nonoccurrence. 

Human reliability analysis is an important component of risk analysis. Reviews of past accidents show that human error accounts for the vast majority of these events. The technique most widely used for estimating human error probabilities is called THERP (Swain and Guttman, 1983). The method uses event trees drawn in a different format to arrive at a human error probability. See Fig. 10.15 for an example. In these event trees, failure paths branch right and success paths branch left. 

There are a number of methods for evaluating the probability of human error. Two of the better-known methods are the Technique for Human Error Rate Prediction (THERP) (Reference NUREG/CR-1278) and the Accident Sequence Evaluation Program Human Reliability Analysis Procedure (Reference NUREG/CR-4772). Error rates are usually established on a per-demand basis. 

Fault trees, failure modes and effects analysis (FMEA), failure modes effects and criticality analysis (FMECA) and event trees use logic, reliability data (component failure rates), and assessed system failure rates, combined with human error failure rates (using methodologies such as HEART or THERP) and other methodologies such as software reliability assessment, to develop estimates of system failure frequencies, and hence plant accident frequencies. 

A THERP tree is a technique used in human reliability assessment to calculate the probability of a human error during the execution of a task. (THERP stands for Technique for Human Error Rate Prediction.) A THERP tree is basically an event tree, where the root is the initiating event and the leaves are the possible outcomes. THERP is described in a publication from 1983 (Swain, A.D. and Guttmann, H.E., Handbook of Human Reliability Analysis with Emphasis on Nuclear Power Plant Applications, NUREG/CR-1278, USNRC), and is still widely used despite its unrealistic assumptions about human performance. One important... 

Two lines of research have addressed the use of BBNs for dependence assessment in HRA. The first one investigates the use of BBNs (Baraldi et al. 2009) as expert models, to increase the repeatability and the transparency of the dependence assessment. The BBN nodes in this study are based on the dependence factors described in THERP (Fig. 4). The lack of traceability in the development of the decision trees used for dependence analysis is one of the main motivations behind the work by Baraldi et al. (2009). The consequence is that the fundamental assumptions behind the influence of factors (e.g. which factors are important and how much each influences the model output) are not directly linkable to the developed model. The work in Baraldi et al. (2009) generally is included in the larger activity of the authors of the present paper to improve trace-ability and transparency of model development in HRA via systematic expert elicitation approaches (cf. Podofillini and Dang, 2010). 

THERP is a complete methodology, from generation of task analyses, through error identification and representation, to quantification of human error probability. The documentation associated with the technique contains methods, models and tables of estimated human error probability to inform the analysis. 

Moreover, an example of an enhanced HRA analysis method has been added, which is intended to complement the existing THERP example in (FAK 2005a). As example of an analysis from past operating experience the fire event at the Forsmark NPP has been chosen. 

THERP involves performing a task analysis to provide a description of performance characteristics of human tasks being analyzed. Results are represented graphically in an HRA event tree, which is a formal representation of the required actions sequence. THERP relies on a large human reliability database containing HEPs, which is based upon both plant data and expert judgments. 

In the process of risk and human reliability assessment, there are various methods to be used, such as Cognitive Reliability and Error Analysis Model (CREAM), A Technique for Human Error Analysis (ATHENA), and Technique for Human Error Rate Prediction (THERP). 

ASEP provides a shorter route to human reliability analysis than THERP by requiring less training to use the tool, less expertise for screening estimates, and less time to complete the analysis. 

Human actions are important contributors to safety therefore, it is important to perform a realistic human response analysis (HRA) using state-of-the-art methodology (Reference 5). The predominant technique used in the PRA was the THERP method, which is appropriate when the operators do not have extensive diagnostic tasks to perform. 

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