Hierarchical task analysis described

Hierarchical Task Analysis commences by stating the overall objective that the person has to achieve. This is then redescribed into a set of suboperations and the plan specifying when they are carried out. The plan is an essential component of HTA since it describes the information sources that the worker must attend to, in order to signal the need for various activities. Each suboperation can be redescribed further if the analyst requires, again in terms of other operations and plans. 

The task analysis techniques described in the previous section are mainly oriented toward observable actions, although hierarchical task analysis (HTA) allows it to address functional requirements as well as the specific actions that are required to satisfy these requirements. 

The particular type of task analysis used in this example is hierarchical task analysis (HTA) (see Chapter 4). This has the advantage that it has been applied extensively in the chemical and other industries. As described in Chapter 4, HTA breaks down the overall objective of a task by successively describing it in increasing detail, to whatever level of description is required by the analysis. At each of the levels, a "plan" is produced that describes how the steps or functions at that level are to be executed. 

This section illustrates how the techniques described in Chapter 4 can be used to develop a procedure for the job of the top floor operator in the batch plant considered earlier. Two techniques are illustrated (i) a hierarchical task analysis (HTA) of the job, and (ii) a predictive human error analysis (PHEA) of the operations involved. HTA provides a description of how the job is actually done while PHEA identifies critical errors which can have an impact on the system in terms of safety or quality. The basic structure of the procedure is derived from the HTA which specifies in increasing detail the goals to be achieved. To emphasize critical task steps, various warnings and cautions can be issued based on the likely errors and recovery points generated by the PHEA. 

The human factors literature is rich in task analysis techniques for situations and jobs requiring rule-based behavior (e.g., Kirwan and Ainsworth 1992). Some of these techniques can also be used for the analysis of cognitive tasks where weU-practiced work methods must be adapted to task variations and new circumstances. This can be achieved provided that task analysis goes beyond the recommended work methods and explores task variations that can cause failures of human performance. Hierarchical task analysis (Shepherd 1989), for instance, can be used to describe how operators set goals and plan their activities in terms of work methods, antecedent conditions, and expected feedback. When the analysis is expanded to cover not only normal situations but also task variations or changes in circumstances, it would be possible to record possible ways in which humans may fail and how they could recover from errors. Table 2 shows an analysis of a process control task where operators start up an oil refinery furnace. This is a safety-critical task because many safety systems are on manual mode, radio communications between control room and on-site personnel are intensive, side effects are not visible (e.g., accumulation of fuel in the fire box), and errors can lead to furnace explosions. 

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