ANALYZING GRAPHIC AND TEXTUAL LAYOUTS WITH GOMS

Mei C. Chuah, Bonnie E. John, John F. Pane
School of Computer Science
Carnegie Mellon University
Pittsburgh, PA, 15213, USA
Tel: +1-412-268-3041
E-mail: {mei+,bej,pane+}@cs.cmu.edu

ABSTRACT

We conducted a preliminary analysis on graphic and textual layouts using GOMS. In the analysis, an airline reservation task was modeled and the task performance times predicted by GOMS were compared with observed empirical data. The results obtained were very promising and show that GOMS may turn out to be a very useful tool for analyzing graphic and textual displays.

INTRODUCTION

Previous studies have shown GOMS to be a powerful and accurate method of analysis for human performance. Despite the wealth of studies that have been performed with GOMS, there has been little or no effort directed towards applying it as a method of analysis for graphic and textual layouts. The primary reason for this is because the syntax of GOMS was not rich enough to model human interaction with the different layout schemes. Recently however, GOMS has been expanded to accommodate low level perceptual, cognitive and motor operators as is shown in [5]. These developments open up the possibility of using GOMS to predict and compare the layout of various displays. Such an analysis would be very useful in testing newly developed user interfaces. In addition, it can also be used to provide heuristics and highlight problems in automatic visualization systems.

KEYWORDS :

GOMS, low level operators, graphic and textual layout, automatic visualization systems.

THE TASK

GOMS is used here to model users' interaction with various kinds of graphical presentations of information while they perform an airline reservation task. The task and graphical layouts are taken from [3], [4]. The task is to find a pair of connecting flights that travel from Pittsburgh to Mexico City, such that the flights are available, the layover is no longer than four hours, and the combined cost of the flights do not exceed $500. Casner presents four examples of graphical layouts that present the required information: 1)tabular layout, where the rows contain information for each flight and the columns represent origin, destination, availability, price, departure time and arrival time; 2)graphical layout that displays cost and availability as text inside rectangles that represent the flights. The right-edge of the rectangles encode departure times, the left-edge encode arrival times and the length of the rectangles encode flight duration; 3)graphical layout that is similar to (2) but uses shading to indicate availability and finally 4)graphical layout that is similar to (2) but uses shading to indicate availability and height to indicate price.1

THE MODEL

There are many different correct algorithms for finishing the task specified above. In [3], Casner examines eight different algorithms for the task. These algorithms capture different types of spatial searches but by no means exhaust all possible task methods. A complete analysis of the layouts would try to identify and model a reasonable set of the possible algorithms as [6] did. However, to limit the scope of this preliminary analysis, a single straightforward, efficient algorithm was used for each layout. From the layout algorithms, we generated a series of CPM-GOMS operators that the model uses to solve the task. In particular, the most important task operation, the visual search, was modeled with the combinations of cognitive, motor and perceptual operators proposed in [5]. Other task operations were broken down into CPM-GOMS operators in similar ways. Each CPM-GOMS operator was assigned a duration based on previous psychological studies [2].

Assumptions about the necessity of eye-movements were based on Tullis' proposal that groups of information within 5 degrees of visual angle can be perceived in a single eye fixation. Casner's users were using a 9x12 inch screen. Assuming that the user was approximately 15 inches from the screen, 5 degrees of visual angles is equal to 0.65 inches, which corresponds to approximately six 12 point characters. In addition it is also assumed that the user always starts scanning from the top-left corner of the layout. This is based on Western convention of reading from left to right, from top to bottom. Such an assumption may not be suitable for users of other cultures. It is also assumed that finger pointing used during the task as an aid to avoid unnecessary visual search. This behavior was observed by Casner during his empirical study.

RESULTS AND CONCLUSION

The performance time predictions made by the GOMS model are given in Table1, together with the user performance times collected by Casner. Using the percentage difference figures from the last row in Table1, we calculated the average absolute % error to be 8.31. This relatively figure indicates that the total task performance time predicted by the GOMS model match quite closely with the empirical timings collected by Casner. Figure1 gives a more intuitive representation of this result.
Table 1

Table 1: Total task performance time as predicted by GOMS and as obtained by Casner's empirical test

Figure 1

Figure1: Comparison between the observed performance time and the GOMS predicted performance time.

Table1 also shows that the predicted performance time is most inaccurate for Layout1. It is hypothesized that one of the primary reasons that the GOMS model under predicted for Layout1 is because it did not take into account problems that users might have with remembering several complicated pieces of information at any one time. From the model, it was determined that in Layout1, a user would have to keep a maximum of 9 chunks of information (4 chunks for the task constraints and 5 chunks for intermediate results) at any one time. This exceeds the average short-term memory capacity that is specified in [2] to be 7 chunks. In the other three layouts however, the user only has to keep a maximum of 6 chunks of information (4 chunks for the task constraints and 2 chunks for intermediate results) in short-term memory throughout the task. If the procedure of Layout1 were changed so that it is less taxing to the user's short-term memory, then the predicted task performance time would be higher and would better match the empirical results. It should be noted that the analysis given here is only a preliminary analysis. First of all, only one procedure was modeled, even though there are many possible procedures that users can use to solve the problem. In addition, Casner's empirical results gave average performance times of several different data sets, whereas only one such data set was modeled here. Nevertheless, the predictions of the GOMS model obtained are very encouraging, With further modeling and empirical validation, we believe that GOMS will be able to provide relatively accurate measures for various graphic and textual layouts.

ACKNOWLEDGMENTS

We would like to thank Steve Roth of CMU for pointing us to Casner's thesis as a domain of study.

REFERENCES

  1. Bovair, S, Kieras, D. E., and Polson, P. G. The acquisition and performance of text-editing skill: A cognitive complexity analysis. Human Computer Interaction, 5, pp. 1-48, 1990.
  2. Card, S.K., Moran, T.P., and Newell, A., The Psychology of Human-Computer Interaction, Erlbaum, Hillsdale, NJ, pp. 139--191, 1983.
  3. Casner, S.M. and Larkin J.H., Cognitive Efficiency Considerations for Good Graphic Design. In Cognitive Science Society Proceedings, Ann Arbor, Michigan, pp. 275-282, Erlbaum, Hillsdale, NJ, 1989.
  4. Casner, S.M., Task Analytic Design of Graphic Presentations. Submitted to the Graduate Faculty of Arts and Sciences in partial fulfillment of the requirements for the degree of Doctor of Philosophy. University of Pittsburgh, 1990.
  5. John, B., Extensions of GOMS analyses to expert performance requiring perception of dynamic visual and auditory information. In CHI'90 Proceedings, Seattle, WA, pp. 107-115, ACM, NY, 1990.
  6. Tullis, T.S., Optimizing the usability of computer- generated displays. In proceedings of HCIÕ86 Conference on People and Computers: Designing for Usability, London, British Computer Society, 1986.
1 Layout 4 was constructed by Casner's automatic visualization system, BOZ.
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