Technique for Human Error Rate Probability

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 (Technique for Human Error Rate Probability)... 

TECHNIQUE FOR HUMAN ERROR RATE PROBABILITY (THERP)... 

Technique for human error-rate prediction (THERP) Structure methodology for modeling human error and task completion in terms of probabilities Predicts human error and task completion probabiUties using a human performance database and expert judgments... 

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. 

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. 

There are a number of methods for evaluation of the probability of human error, for example, the technique for human error rate prediction, discussed earlier (Clause 6.2.1 of Chapter V). The best source for determining the human error rate would be company/facility-specific historical data, but in most organizations this is not available [11]. So, other means need to be explored. The reliability of support systems necessary for an operator s action is also an important issue that can influence risk reduction. The majority of SIS systems are designed as deenergize to actuate. The calculation of PFD for these SIS systems does not generally have to take into consideration any system outside of the SIS. See also Clause 3.2.2. 

For some time there has been an interest in exploring the underlying reasons, as well as probabilities, of human error. As a result there are currently several models, each developed by separate groups of analysts working in this field. Estimation methods are described in the UKAEA document SRDA-Rll, 1995. The better known are HEART (Human Error Assessment and Reduction Technique), THERP (Technique for Human Error Rate Prediction), and TESEO (Empirical Technique to Estimate Operator Errors). 

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... 

The Technique for Human Error Rate Prediction (THERP) was developed by Swain and Guttman (1983) to evaluate the probability of human error within specific tasks. THERP uses a fault tree approach to model Human Error Probabilities (HEP), but also attempts to account for other factors in the environment that may influence these probabilities. These factors are referred to as Performance Shaping Eactors (PSE). The probabilities used in THERP can either be generated by the analyst, usually from simulator data, or can be taken from tables generated by Swain and Guttman from available data and expert judgement. 

Technique for Human Error Rate Prediction, (THERP), for the response phase. The aim of this methodology is to calculate the probability of the successful performance of the necessary activities for the completion of a task. 

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. 

---