Control Task Analysis 8
The efficiency of skilled performance is due to the ability to compose the process needed for a specific task as a sequence of familiar subroutines which are useful in different contexts. This implies the existence of links in the sequence at standard key nodes or "states of knowledge" which are characteristic of the specific skill. The data process stops at such links, the mode of processing and frequently the level of abstraction changes, and to study and identify the processes, the activity must be structured according to such key nodes.
-- Rasmussen ( 1976, p. 374)
Our goal in this chapter is to explain the second phase of CWA, control task analysis, in full detail.1 As we explained in chapter 3, control task analysis and work domain analysis are complementary. Although an analysis of control tasks does not identify the support required to deal with unanticipated events, it does allow us to identify the requirements associated with known, recurring classes of situations. Such an analysis can identify the constraints on what needs to be done, independently of how or by whom. We describe the decision ladder ( Rasmussen, 1974, 1976), a modeling tool that can be used to develop control task models. An example of how the decision ladder can be used to conduct a control task analysis is presented for the DURESS II process control case study introduced in chapter 6. By the end of this chapter, you should understand how control task constraints both inherit, and build on, the work domain constraints discussed in the previous chapter.
The need for some kind of task analysis was described in detail in chapter 3. We outlined three kinds of approaches: input-output, sequential flow, and timeline. The first is an example of constraint-based analysis, whereas the latter two are examples of instruction-based analysis. We went on to show that a constraint-based approach is required to deal with open systems. Instruction-based approaches do not have the flexibility required to deal with substantial disturbances. Thus, the input-output analysis is the most appropriate for complex sociotechnical systems.____________________