Behavioral and Physiological Metrics - Measuring the User Experience: Collecting, Analyzing, and Presenting Usability Metrics - page 175

conclusion based on a few heat maps. Here are a few other important tips to
keep in mind as you dive into the data.

• Controltheamountofexposuretimeforeachparticipant.Iftheydidnot

see the same image or stimuli for the same time, predefine the time to
only include the first 10 or 15 seconds, or whatever duration makes the
most sense given the context.

• Ifyouarenotabletocontrolforexposuretime,analyzethedwelltime

as a percentage, not as an absolute. If someone spent 10 seconds and the
other person spent 1 minute, their eye movements will be different, as
well as the actual amount of time spent looking at each element.

• Onlylookattimedatawhentheparticipantisengagedwiththetask.Do

not include any time data when the participant is debriefing about her
experience and still being tracked.

• During the study, make sure that the participants are being tracked.

Monitor their eye movements in real time. As soon as they start to slouch
or turn their head, remind them gently to maintain their original position.

• Becarefulwhenanalyzingeyemovementsondynamicwebsites.Websites

that change considerably due to ads, flash, frames, and so on confuse
most eye-tracking systems. Every new image is essentially treated as sepa-
rate stimuli. We strong recommend that you consolidate as many web
pages together as possible, knowing that not every page is exactly identi-
cal. Otherwise, you will end up with way too many web pages that were
only viewed by a single participant. An alternative to this is to simply
use static images. They are much easier to analyze, but lack an interactive
experience.

• ConsiderusingatriggerAOItocontrolwhereparticipantsareinitially

looking at the start of the experiment. A trigger might say “look here
to start the experiment.” The text might be in the middle part of the
page. After the participant has fixated on the text for a certain number
of seconds, the experiment begins. This means that all participants start
looking from the same location. This might be overkill for the typical
usability test, but should be considered for more tightly controlled eye-
tracking studies.

7.2.6 Pupillary Response
Closely related to the use of eye tracking in usability studies is the use of infor-
mation about the response of the pupil. Most eye-tracking systems must detect
the location of the participant's pupil and calculate its diameter to determine
where he or she is looking. Consequently, information about pupil diameter
is included in most eye-tracking systems. The study of pupillary response, or
the contractions and dilations of the pupil, is called pupillometry. Most people
know that the pupil contracts and dilates in response to the level of ambient
light, but many people don't know that it also responds to cognitive processing,
arousal, and increased interest. Typically the greater the level of arousal or inter-
est, the larger the pupil size.