An Exploration of Techniques to Improve Relative Distance
Judgments within an Exocentric Display
In the first and second experiments it was observed that subjects took on average between 31 to 34 seconds to make an accurate relative distance judgment using a manual rotation technique. And from the second experiment, it was quite clear that subjects not only performed well, but strongly preferred, a rotation technique which gave them both manual control and quick access to the key views of their visual scene. What remains to be determined now, however, is what minimal length of time is necessary for rotation to be effective in making these relative distance decisions.
The purpose of this next experiment, run as a pilot study, was to estimate at what point the use of Manual Rotation, in conjunction with Discrete Views, would no longer be an effective technique for determining relative distances, due to time constraints. This particular rotation technique was selected for this experiment because subjects, when using this technique, were able to make relative distance judgments with very high accuracy. In addition, subjects preferred this method over the other rotation techniques, and it seemed to allow the implementation of a range of distance judgment strategies. The reason time constraints are being addressed is that some applications could require very rapid decision-making. In a command-and-control situation, for example, a military commander may not have 30 seconds to determine which enemy is closest to one of his or her friendlies. Thus, the goal of this next experiment is to provide a guideline as to the minimal time required in order to make a relative distance judgment using the Discrete View + Manual Rotation technique.
The major difference between this and the prior two experiments is that time is now an independent variable. Subjects were all given trials with three different time allotments for making their relative distance judgments. The time allotments chosen were 6, 12 and 24 seconds. This scheme covered a reasonable range of decision duration. In the first two experiments it took subjects, on average, 32 seconds to make a relative distance judgment using a technique which incorporated only manual rotation. Yet it was also shown that subjects took, on average, 24 seconds to make (comparably good) judgments with only the Discrete Views technique. Thus for this experiment, subjects should still be able to make relatively accurate judgments within the 24-second period. For the shorter time durations, however, their performance was not predicted. The time allotment of 12 seconds is also of interest because this is approximately the amount of time that subjects took to make a relative distance judgment without image rotation, in the first experiment.
For the third experiment, subjects were again shown four differently colored cubes (yellow, blue, green and lavender) hovering over a terrain, and were asked to indicate, via the keyboard, "which colored cube is closest to the white cube" within the scene. The apparatus, both in terms of hardware and software, was identical to the second experiment. The same 18 judgments used per condition in the previous two experiments were again used as the 18 judgments for each condition in experiment three. The geometric field of view and initial eyepoint elevation angle were also the same as in the other experiments.
5.3 Experimental Design and Procedure
This third experiment used a 3 X 1 within subjects design. The three treatment conditions were the time allocations for making relative distance judgments in an exocentric display (6, 12 and 24 seconds) using the Discrete Views + Manual Rotation technique. Three subjects were run; all were males ranging in age from 21 to 29. Subjects for this experiment were chosen from those that had participated in the second experiment. Since these subjects were already familiar with this technique, it was felt that they would be able to more efficiently practice making judgments under the time constraints. The order of the three treatments was counterbalanced across subjects. Each treatment contained the same 18 object position configurations. The order of the 18 trials was randomized for each treatment so that subjects would not memorize response orders. Also, the color assigned to the colored cubes was rotated in order to again discourage recall of previously seen configurations and to guard against response biases due to chromostereopsis.
In this experimental procedure, the experiment overview was read to each subject. Subjects were instructed to make their judgments as accurately as possible within the time allotted; they were also allowed to make judgments in less time than the condition specified. Subjects were then seated at the computer display 57 cm away from the monitor. The three different time intervals were explained to the subjects, and they were allowed to practice making relative distance judgments using each of the different time allotments. The practice session judgments covered a range of difficulty so that subjects would be better prepared for the experimental conditions. Subjects were not given feedback on their judgments.
At the beginning of each treatment condition, subjects were told how much time they would have in order to make a single judgment. For each trial, subjects were given a two- second warning before the end of the time allotment. At the end of the appropriate time period, the screen was covered, and the subject was asked to make their decision. At the end of each treatment, subjects were asked to rate their confidence in the accuracy of their responses in the latest condition on a scale from 1 to 10, where 1 indicated that they had no confidence in their judgments and 10 indicated that they were extremely confident in their judgments. After completing all treatment conditions, subjects were asked to fill out a post-experiment questionnaire.
For each treatment, the following data were calculated for each subject:
1) the subject's "Accuracy" as the percentage of correct judgments made within the treatment condition.
2) "Views" as the average number of times they used any of the discrete views during a single judgment.
3) "Confidence" as the rating on a scale from 1 to 10 that each subject gave at the end of each treatment, indicating how confident they were in their answers.
Figure 5.1 shows each Subject's Accuracy for each of the time allotments.
Figure 5.1: Subjects' Accuracy per Time Allotment for Experiment 3
As seen in the above figure, all subjects performed well, as expected, within the 24 second time allotment. What is surprising, however, is the ability of both Subjects 2 and 3 to also perform well in the reduced time allotments. In particular, Subject 3 performed as well within the six second time allotment as did subjects in the previous two experiments - who, on average, took over 30 seconds to make their judgments.
Figure 5.2 shows the mean accuracy, for the three pilot subjects combined, for each of the time allotments.
Figure 5.2: Mean Accuracy per Time Allotment for Experiment 3
On average we see a decrease in subject's accuracy as the time allotted to make a relative distance decision is reduced. A planned comparison showed a significant difference in accuracy when subjects were given 24 seconds to make a judgment when compared to twelve and six seconds (F = 3.24, df = 6, p < .018). When given 24 seconds to make a decision, with an average accuracy of 85, subjects are well above the approximate mean accuracy rate of 73 percent, as seen in experiments one and two. When the time is reduced to either twelve or six seconds, the accuracy falls below this mark.
Subjects also varied in the amount they actually used the Discrete Views capabilities, as indicated by the "Views" data. As seen in Figure 5.3, Subject 1 seemed to make greater use of the views as opposed to Subjects 2 and 3. Subject 3 did not use the different discrete views at all, but merely rotated around the initial discrete view.
Figure 5.3: Average Subject Use of Discrete Views per Judgment
These results suggest that there is wide subject variability in making rapid relative distance judgments. For some, making a very fast relative distance judgment, using the rotation technique provided, was quite easy; yet others required longer display time in order to make an accurate judgment. Given enough time, however, all subjects were able to make quite accurate judgments.
Consistent with the previous experiments, subjects used a variety of techniques in order to make their judgments. One subject relied heavily upon the discrete views capability, while another subject chose only to rotate the image around a single axis. This subject stated that he was primarily using the motion parallax information provided by the relative movements of objects as they circled around the white cube. Another subject felt that he used both techniques to varying degrees depending on the amount of time available.