Hierarchical Task Analysis

Hierarchical Task Analysis (HTA) is a method for understanding the hierarchical decomposition of system goals, based on a desired objective or end state (Annett and Stanton 1998). Although HTA originated from scientific management methods and is often employed to decompose the tasks of individual operators (Stanton 2006), it is ultimately a systems method, enabling analysts to decompose the whole system in terms of its goals, sub-goals and requisite activities. It provides a normative model of functioning, that is, it describes what should be done to achieve the overall goals of the system.

The HTA method was developed in response to a need to better understand cognitive tasks brought about by the changing nature of industrial work processes during the 1950s and 1960s (Annett 2004). At the time, the focus on cognition as well as physical work made it unique, and it subsequently became arguably the most popular of all human factors methods (Stanton 2006).

HTA works by decomposing systems into a hierarchy of goals, sub-ordinate goals, operations and plans; it focuses on ‘what an operator ... is required to do, in terms of actions and/or cognitive processes to achieve a system goal’ (Kirwan and Ainsworth 1992). It is important to note here that an ‘operator’ may be a human or a technological operator (e.g. system artefacts such as equipment, devices and interfaces). HTA outputs therefore specify the overall goal of a system, the sub-goals to be undertaken to achieve this goal, the operations required to achieve each of the sub-goals specified and the plans that are used to ensure that the goals are achieved. The plans component of HTA is especially important as they specify the sequence, and under what conditions, different sub-goals must be achieved to satisfy the requirements of a super-ordinate goal.

HTA has been widely used in several domains, including the process control and power generation industries, the military (Ainsworth and Marshall 1998, Kirwan and Ainsworth 1992) and transport, including aviation (Stanton et al. 2009) and bus driving (Salmon et al. 2011). It has also been adapted for use in a range of human factors applications, including training (Shepherd 2002), design (Lim and Long 1994), error and risk analysis (Baber and Stanton 1994) and the identification and assessment of team skills (Annett et al. 2000).

BOX 2.9 HIERARCHICAL TASK ANALYSIS FOR RAIL LEVEL CROSSING SYSTEMS

We used HTA to understand the rail level crossing system, in terms of the goals, sub-goals and operations of the key actors, including road users, train drivers, the train and the rail level crossing infrastructure. See Chapter 5 for further information.

 
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