The first generation HRA methods have been strongly influenced by the viewpoint of probabilistic safety assessment (PSA) and have identified man as a mechanical component, thus losing all aspects of dynamic interaction with the working environment, both as a physical environment and as a social environment 1. In many of these methods – such as Technique for Human Error Rate Prediction (THERP)2, 3, 4, 5, 6. Accident Sequence Evaluation Program (ASEP)7, and Human Cognition Reliability (HCR)8 – the basic assumption is that because humans have natural deficiencies, humans logically fail to perform tasks, just as do mechanical or electrical components. Thus, HEP can be assigned based on the characteristics of the operator’s task and then modified by performance shaping factors (PSF). In the first HRA generation, the characteristics of a task, represented by HEPs, are regarded as major factors; the context, which is represented by PSFs, is considered a minor factor in estimating the probability of human failure9 .This generation concentrated towards quantification, in terms of success/failure of the action, with less attention to the depth of the causes and reasons of human behaviour, borrowed from the behavioural sciences10.
THERP and approaches developed in parallel – as HCR, developed by Hannaman, Spurgin, and Lukic in 1985 – describe the cognitive aspects of operator’s performance with cognitive modelling of human behaviour, known as model skill-rule-knowledge (SKR) by Rasmussen (1984)11.This model is based on classification of human behaviour divided into skill-based, rule-based, and knowledge-based, compared to the cognitive level used (see Figure 9.1Figure 9.1
The attention and conscious thought that an individual gives to activities taking place decreases moving from the third to first level. This behaviour model fits very well with the theory of the human error in Reason (1990), according to which there are several types of errors, depending on which result from actions implemented according to the intentions or les＿ 12.Reason distinguishes between: slips, intended as execution errors that occur at the level of skill; lapses, that is, errors in execution caused by a failure of memory; and mistakes, errors committed during the practical implementation of the action. In THERP, instead, wrong actions are divided into errors of omission and errors of commission, which represent, respectively, the lack of realisation of operations required to achieve the result and the execution of an operation, not related to that request, which prevents the obtainment of the result13, 14.
The main characteristics of the methods can be summarised as follows15 .
- Binary representation of human actions (success/failure);
- Attention on the phenomenology of human action;
- Low concentration on human cognitive actions (lack of a cognitive model);
- Emphasis on quantifying the likelihood of incorrect performance of human actions;
- Dichotomy between errors of omission and commission;
- Indirect treatment of context.
Among the first generation techniques are: absolute probability judgement (APJ), human error assessment and reduction technique (HEART), justified human error data information (JHEDI), probabilistic human reliability analysis (PHRA), operator action tree system (OATS), and success likelihood index method (SLIM)16, 17 .Among these, the most popular and effectively method used is THERP, characterised as other first generation approaches by an accurate mathematical treatment of the probability and error rates, as well as computer programs well-structured for interfacing with the trees for evaluation of human error of a fault event and trees18.The base of THERP is event tree modelling, where each limb represents a combination of human activities, influences upon these activities, and results of these activities19.
The basic analytical tool for the analysis of human reliability is represented with the graphics and symbols in Figure 9.2
First generation HRA methods are demonstrated with experience and use, not able to provide sufficient prevention and adequately perform its duties20. The criticism of base to the adequacy of the traditional
methods is that these approaches have a tendency to be descriptive of events in which only the formal aspects of external behaviour are observed and studied in terms of errors, without
considering reasons and mechanisms that made them level of cognition. These methods ignore the cognitive processes that underlie human performance and, in fact, possess a cognitive model
without adequate human and psychological realism. They are often criticised for not having considered the impact of factors such as environment, organisational factors, and other relevant PSFs;
errors of commission; and for not using proper methods of judging experts 21, 22, 23. Swain remarked that “all of the above HRA
inadequacies often lead to HRA analysts assessing deliberately higher estimates of HEPs and greater uncertainty bounds, to compensate, at least in part, for these problems”24. This is
clearly not a desirable solution.
Despite the criticisms and inefficiencies of some first-generation methods, such as THERP and HCR, they are regularly used in many industrial fields, thanks to their ease of use and highly quantitative aspects.
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