Instrumented Usability Analysis for Mobile Devices - Human-Computer Interaction and Innovation in Handheld, Mobile and Wearable Technologies - page 15

screen). The path that the participants traversed
during the experiment was relatively quiet. There
was therefore little reason for the participants to
speed up or slow down their step rate. In other
experiments where the path is more complex or
the user must perform different tasks, we would
expect the step rate to be more variable, but in
this study, mean step rate might actually have
been sufficient when analysing the effects of
walking rate.

instance there is no simple correlation between
tap error and device acceleration.

Discussion

Using standard usability metrics, we were able to
show that tapping accuracy was, unsurprisingly,
typically greater when sitting still, rather than
walking. However, the above results demonstrate
the extra insights into user behaviour that were
made possible by taking an instrumented usability
approach.
Specific experimental observations of this
instrumented usability approach are:

Other Analysis

The results presented so far have involved ana-
lysing the acceleration trace to extract informa-
tion about the users' steps. Now we examine all
disturbances affecting the device. If the device is
moving around more, we would expect the user
to be tapping less accurately. By looking at the
magnitude of the of the acceleration trace in x , y ,
and z we gain an insight into the mean magnitude
of disturbance that the device was going through.
Figure 16 shows a scatter plot of the magnitude of
the tap error plotted against the mean magnitude
of the disturbance of the device for the one second
previous to the tap. As the figure shows, in this

• Users' tapping time is significantly cor-

related with gait phase angle. Users were
approximately 3 times more likely to tap at
the most favoured tap phase than the least
favoured tap phase.

• Users' tapping position accuracy is sig-

nificantly higher (lower mean error and
lower variability) at these preferred phase
angles. Analysis of the timing data for the
different phase regions showed that users
subconsciously delayed their target selec-
tion in order to tap in one particular phase

Figure 15. The step rate of five typical participants for duration of the walking condition