Page 394 - Sensing, Intelligence, Motion : How Robots and Humans Move in an Unstructured World

RESULTS—EXPERIMENT ONE 369

for either the visible or invisible environments, the path length for the physical
task is not significantly different from the path length in the virtual task. Also,
in either of physical or virtual tasks, the path length in the visible environment
does not significantly differ from the path length in the invisible environment.

7.4.6 Implementation: Two-Way Analysis for Completion Time
In the previous section we have analyzed the effects of test factors on the length
of paths generated by the human subjects in Experiment One. We will now
analyze how these same factors affect another performance indicator, the task
completion time.
Each completion time score is random and independent (for the 48 subjects
tested here); this meets the “sampling assumption” of nonparametric statistics
and analysis of variance. Even though a closer look at the completion time data
shows that they do not obey a normal distribution (as the ANOVA assumption
requires), we still use ANOVA, counting on the F test known to be robust.
To analyze the effect of all factors on the completion time data, a three-way
analysis of variance has been done. The ANOVA variables are as follows:

• Dependent variable: Completion time.
• Independent variables:
1. Direction factor, with two levels: LtoR and RtoL.
2. Visibility factor, with two levels: visible and invisible.
3. Interface factor, with two levels: virtual (simulation) and physical (booth).

Second, since we are more interested in the visibility factor and interface factor,
and since the performance in LtoR task significantly differs from that in RtoL
task, a two-way ANOVA was implemented. The ANOVA variables are:

• Dependent variable: Completion time.
• Independent variables:
1. Visibility factor, with two levels: visible and invisible.
2. Interface factor, with two levels: virtual and physical.

Results. The summary of ANOVA results of analysis of variance for all three
factors used in Experiment One appears in Table 7.11. The p-levels for the
interface factor, the direction factor, and the interaction between them are less
than 0.01. This means that these two main effects likely significantly affect the
dependent variable (completion time), and there is interaction between them. The
p-levels for the remaining main effect, visibility, and for interactions with this
factor are greater than 0.01. This means that there is no significant difference
for these effects and interactions. However, given that an interaction has been
detected, we should not be forming any conclusions from the results in Table 7.11
until we separate the factor levels.