Two Applications of a Critical-Instability Task to Secondary Work Load Research

Secondary (or ``auxiliary,'' or ``subsidiary'') tasks have often been used to load or to stress an operator while he performs a primary manual control task. As discussed in Poulton,
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the secondary task should measurably stress the operator's parameter being tested, without rendering the primary task behavior meaningless. This problem has given rise to several types of secondary tasks, which fall into two categories. In the first category are secondary tasks that do not involve the same form of manual control activity as the primary task, such as: mental arithmetic, verbal report of warning light detection, verbal repetition of heard number sequences, etc.
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In the second category are secondary tasks involving similar psychomotor activity as the primary task, such as: tracking in a second degree-of-freedom, two-handed tracking, monitoring, and extinguishing warning lights, etc.
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In the latter case, the distinctions between a secondary task and a multiloop control situation are not sharp and depend primarily on the relative emphasis placed on secondary task performance, specified by the procedures or practiced by the operator.