Efficiency/Control and Coordination/ placed by Margith Strand/ January 9, 2011

Abstract

Recent work presented at CHI �98 (Computer Human Interaction) included two papers (Wang et al. 1998; Zhai and Milgram 1998) on the topic of measuring human coordination between translational and rotational degrees of freedom. While both papers are attempts at measuring human coordination, the two methods, simultaneity and efficiency are quite different. Both methods are reviewed and it is argued that neither of these two metrics alone entirely captures the concept of coordination. Rather, a true measure of coordination must take into account performance in both the time domain and the space domain. Simultaneity and efficiency each alone only captures one of the two dimensions which encompasses human coordination. A qualitative definition of coordination is proposed and is compared to the related concepts of simultaneity, efficiency, performance, and control.

Introduction: Previous Work on Human Coordination

This paper is hardly a complete review of all previous work on the topic of coordination; rather it is a review of recent papers from the human computer interaction literature. The emphasis is upon the methods used in each paper to quantify coordination. First, two papers, one measuring simultaneity and one measuring the efficiency of users ability to coordinate movement between translational and rotational degrees of freedom, will be reviewed and contrasted. Rather than arguing that simultaneity or inefficiency are incorrect measures of coordination, it is argued that each alone is an incomplete measure.

Simultaneity (Wang et al. 1998)

Wang et al.�s work is an extension of (Jacob et al. 1994) work on the integrality and separability of input devices. Integrality refers to the ability to move diagonally across a multi-dimensional space, while separability describes movement along one degree of freedom at a time. In other words, shortest distance straight-line Euclidean trajectories are evidence of integral movements while "city-block" trajectories are evidence of separable movements, see Figure 1.



Figure 1. Depiction of integral and separable movements. The degrees of freedom "X" and "Y" may represent any two degrees of freedom that are being manipulated.

For a given timeline, it is possible to compute the ratio of Euclidean to city-block movements for a given task. This ratio is a measure of the integrality of a given input device (Jacob et al. 1994), which can then be compared to the integrality ratio of other input devices for the same task. The higher the ratio, the greater the integrality of the device. One example of the use of this measurement has been to demonstrate that users can control three degrees of freedom simultaneously in a two translational and one rotational degree of freedom device, the Rockin�Mouse (Balakrishman et al. 1997).

Essentially, integrality is a measure of the simultaneity of motion among multiple degrees of freedom. The Jacob et al. method of measuring integration looks at movement that is greater than a fixed threshold in order to filter out very small movements. Other than the fixed threshold, integration/ simultaneity is measurement in the time domain only, and says nothing about magnitude or the direction of the movement. Neither Jacob et al. nor Wang et al. has made the claim that integrality is a measure of coordination. However, they are clearly related concepts. Exactly how does integrality differ from an "ideal" measure of coordination (which has yet to be defined)?

Selecting different constants for the different threshold parameters in the integration measure can lead to different results. Ideally, a measure of coordination should not be a function of constants chosen by the experimenter.
Just because there is motion in all degrees of freedom does not necessarily mean that the motion is contributing towards reaching the goal. For example, randomly generated movements above the threshold would qualify as integrated motion but should not be consider as coordinated motion.
Sometimes not moving in one degree of freedom is just as important as movement in another degree of freedom. It is possible to imagine a task where city-block motion is required for all or part of the task. The measure of integration is independent of the task, it only checks whether there is motion in more one dimension regardless of whether that motion is required or not. A measure of coordination should be a function of the task. (Consider, for example, motion in a circle within an environment which has two degrees of freedom. The amount of movement on each degree of freedom will range from zero to maximum, and only a task dependent measure of coordination can possibly track these changes in magnitude.)