Instrumented Usability Analysis for Mobile Devices - Human-Computer Interaction and Innovation in Handheld, Mobile and Wearable Technologies - page 6

of tapping buttons or other widgets on a touch
screen. This is a common form of input, and is
effective when seated, but difficult when walk-
ing. (Brewster, 2002) showed a more than 30%
reduction in performance tapping buttons on the
display of a PDA when walking, compared to
sitting. If we can gain more detailed insight into
how and when users tap during walking, we might
be able to adjust the design of the interface to
improve robustness.
Here we show how sensors, like accelerom-
eters, can be used in ways other than for explicit
interaction. In this case we use the acceleration
data to infer the user's gait, and we investigate
whether the rhythm of walking affects the tap
timing and error rate of a user selecting targets
on screen, while walking and sitting.

accelerometer was used to detect movement of the
device, sampling at a rate of approximately 90Hz.

Task

The interface used for the study is displayed in
Figure 4. Participants were asked to tap on a series
of cross-hair targets (drawn 30 pixels high and
wide) that were displayed on the screen. There
were 15 possible target positions spaced equally
around a 3 wide by 5 high grid of positions on
the screen. Every second target presented to the
participants was the target in the centre of the
screen. This ensured that the user must return the
stylus to the centre of the screen such that when
a target other than the central target was tapped,
the path to that target was always from the centre.
The other 14 targets were displayed to the user in
a random order four times each. The accuracy and
speed of tapping were both emphasised as equally
important. The position of the tap was recorded
as the initial stylus down position on the screen.
Once one target had been selected, the next target
was displayed a random time interval from 0.5

EXPERIMENT

Introduction

This study examines in detail the behaviour or
users tapping on the screen of a mobile device.
It analyses behaviour in two different situations
that a user might perform this task: while sitting
and while walking. The sitting condition will be
used to provide a performance baseline for the
walking condition. Disturbances to the device and
the users stylus due to the user's walking will af-
fect how and how well the user performs the task.
By instrumenting the device with a sensor (in this
case an accelerometer), we will be able to gain a
deeper understanding of how these disturbances
affect performance.

Figure 4. PDA with the Xsens P3C accelerometer
attached to the serial port

Equipment

This system was developed using an HP 5550
PDA with the Xsens P3C 3 degree of freedom
linear accelerometer attached to the serial port,
as shown in Figure 4. Its effect on the balance of
the device is negligible (its weight is 10.35g). The