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Figure 16. A scatter plot of the magnitude of the device disturbance plotted against the tap error in pixels
area rather than any other. There is further
structure in the left step-right step tap den-
sity, error biases and variability, but even
when averaged over all steps, the results
are significant.
the device while tapping, which minimised the
effect of the external disturbance in this instance,
so the main disturbance came from the gait cycle
of walking itself.
•
The distribution of tapping errors varies
both with phase of step, and between walk-
ing and sitting and across different screen
positions.
CONCLUSION AND FUTURE WORK
This work has demonstrated that by making
fine-grained observations from sensors during a
usability study, that we can learn increased detail
about the timing and error rates for users. Until
now, linking the analysis of, for example walking
behaviour, in a realistic setting would typically
have required the use of hand scoring videotapes of
users' actions—a time-consuming, and potentially
subjective and error-prone approach which is also
not open to online experimental control. Recent
rapid developments in mobile device capacity,
and compact sensors, coupled with the use of the
analytic tools from synchronization theory, have
opened up a new way of investigating gait effects
in interaction. The inertial sensors monitor walk-
ing patterns throughout the experiment, and can
potentially be used together with machine learning
It is interesting to note that although there is no
simple correlation between tap error and device
acceleration, the inferred phase angle, which is
based solely on the acceleration observations,
does show a strong link between acceleration
and tapping accuracy, emphasizing the need for
appropriate models in data analysis. One potential
reason for this is that the walking route chosen
for the study did not require the user to make ir-
regular adjustments to their movements. The path
was quiet so that the user only infrequently had to
avoid objects. This limited the disturbance of the
device to lower than might be expected in a more
crowded environment, or in e.g. a moving vehicle.
The participants grounded their tapping hand on
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