Behavioral and Physiological Metrics - Measuring the User Experience: Collecting, Analyzing, and Presenting Usability Metrics - page 166

infrared light sources create reflections on the surface of the participant's eye
(called the corneal reflection), and the system compares the location of that
reflection to the location of the participant's pupil. The location of the corneal
reflection relative to the pupil changes as the participant moves his eyes.
The first activity in any eye-tracking study is to calibrate the system by ask-
ing the participant to look at a series of known points; then the system can
subsequently interpolate where he is looking based on the location of the cor-
neal reflection (see Figure 7.3 ). Typically the researcher can check the quality
of the calibration, usually expressed as degrees that deviate from the X and Y
visual planes. Deviations less than one degree are generally considered to be
acceptable, and less than one-half of a degree is very good. It is critical that the
calibration is satisfactory; otherwise all the eye movement data should not be
recorded or analyzed. Without a good calibration there will be a disconnect
between what the participant is actually looking at and what you assume he
is looking at. Following calibration, the moderator makes sure the eye move-
ment data are being recorded. The biggest issue tends to be participants who
move around in their seat. Occasionally the moderator is required to ask the
Figure 7.3 An example of SMI software used to run an eye-tracking study and monitor eye movements in
real time. The three windows contain study details (left), stimuli being tracked (top right), and eye being
tracked (bottom right).