Task characteristics, performance-influencing factors

In the previous chapter, a comprehensive description was provided, from four complementary perspectives, of the process of how human errors arise during the tasks typically carried out in the chemical process industry (CPI). In other words, the primary concern was with the process of error causation. In this chapter the emphasis will be on the why of error causation. In terms of the system-induced error model presented in Chapter 1, errors can be seen as arising from the conjunction of an error inducing environment, the intrinsic error tendencies of the human and some initiating event which triggers the error sequence from this imstable situation (see Figure 1.5, Chapter 1). This error sequence may then go on to lead to an accident if no barrier or recovery process intervenes. Chapter 2 describes in detail the characteristics of the basic human error tendencies. Chapter 3 describes factors which combine with these tendencies to create the error-likely situation. These factors are called performance-influencing factors or PIFs. 

The variability of human performance is reflected by individual differences of skill, experience, motivation, and other personal characteristics of work force. There can be a wide range of specific environmental situations and other physical aspects of the tasks to be performed. Only some of this variation regarding the performance-shaping factors is accounted for in HEP s by provision of different estimates of HEP s for different sets of influencing factors such as experience of the operator, level of stress, and ergonomic layout. 

Human error tmalysis (HEA) is a systematic evaluation of the factors tliat influence tlie performance of human operators, maintenance staff, teclmicians, and otlier persomiel in tlie plant. HEA involves the evaluation of one of several types of task analysis, which is a metliod for describing tlie physical and enviromiiental characteristics of a task along witli tlie skills, knowledge, and capabilities required of tliose who perform the task. Tliis type of analysis can identify error-likely situations tliat can cause or lead to an accident. 

Man-machine interface consists of such aspects as allocation of functions (man vs. machine), automation, accessibility, human tasks, stress characteristics, information presented to the human, and the reliability of interfaces coupled with the decisions on the basis of such information. Both human and machine elements of a system can fail, and their failures have varying effects on the system s performance. Some human errors cause total system failute or increase the risk of such failure. Human factors exert a strong influence on the design and ultimate reliability of a system (Kirwan 1994). 

---