Tracking in Two Dimensions as a Function of Dimension Priorities and Tracking Difficulty

Abstract

The present paper develops the argument that an effective evaluation of performance under time-sharing conditions requires a joint manipulation of tasks difficulty and operator's resources allocation. An experiment is presented in which each of the dimensions in a two dimensional pursuit tracking task waS manipulated and controlled seperately. Single and dual task conditions were created by presenting one dimension or two dimensions sL~ultaneously. Time-sharing efficiency was assessed under a joint manipulation of tracking difficulty on each dimension and their relative priorities. Subjects' tracking ability was individually calibrated by adaptive procedures. Regression equations and performance functions wers obtained to describe the joint effects of the experimental variables. Results are discussed in terms of their implications to the problem of measuring capacity, and their contribution to the understanding of tracking behavior. Performance on any given task can be expressed as a function of two sets of variables. One set includes such variables as the sensory quality of stimuli; predictability of stimulil availlability and completeness of relevant memory codes, SR compatibility, response complexity, amount of practice etc. etc. Norman and Bobrow (1975) seem to subsume all those parameters under the title data aualitv. As we feel that the connotation of this term is too limitted, we prefer the name Subiect-TRBk-ParSlllleters (m). The STP are the constraints imposed on performance by the task (or more percisely, the encounter of a specific task and an individual subject). Wi thin those constraints the human operator being an aCtive system is free to mobilize his various resources. These resources are the second set of variables which determin the level of task performance.In concurrent performance the joint demands of tasks very often exceed the ability of the operator to supply resources. When this happens an internally or externally determined "tradeoff" or "Priorities" function must be established to efficiently control the allocation of resources. Norman and Bobrow (1975) propose the term Performance Operating Characteristics (PCC) curves, to describe the tradeoff of performance levels on two tasks under time sharing conditione as a function of the relative allocation of resources. A whole family of different poe curves, each representing one STP combination, can be created for any pair of tasks.If thia approach is accepted, it is apparent that a comprehansive description of performance limitations under time-sharing conditions on any given pair of tasks, requires a joint assessment of the tradeoff function between task difficult,y 118 and resource allocation. This requirement represents a marked deviation from the traditional secondary task techniques that completely ignore the manipulation of resource allocation and concentrate exclusively on STP manipulations. We therefore, percieve the pressnt study as a pioneering effort towards the development of methodological techniques that enable a joint manipula...