Behavioral and Physiological Metrics - Measuring the User Experience: Collecting, Analyzing, and Presenting Usability Metrics - page 173

associated with specific AOIs. Figure 7.11 is an
example of the type of metrics derived from a
single AOI.

DWELL TIME
Dwell time is the total amount of time spent
looking within an AOI. This includes all fix-
ations and saccades within the AOI, includ-
ing revisits. Dwell time is an excellent metric
that conveys the level of interest with a certain
AOI. Obviously, the greater the dwell time,
the greater the level of interest in the AOI. As
a general rule of thumb, dwell times less than
100 msec generally mean the participant processed a limited amount of infor-
mation. A dwell time greater than 500 msec generally means the participant had
an opportunity to process the information.

Figure 7.11 Example of common metrics calculated for a single AOI
using the SMI software.

NUMBER OF FIXATIONS
The number of fixations is simply the total count of fixations with an AOI. The
number of fixations, as expected, is strongly correlated with dwell time. Because
of this, we typically just report dwell time.

FIXATION DURATION
Fixation duration is the average time for fixations. Fixation duration typically
ranges from 150 to 300 msec. Fixation duration, similar to number of fixations
and dwell time, represents the relative engagement with the object. The greater
the average fixation duration, the greater the level of engagement.

SEQUENCE
The sequence represents the order or sequence in which each AOI is first fixated.
The sequence tells the researcher the relative prominence of each AOI within
the context of a given task. Sometimes it is very helpful to know which AOIs are
jumping out to users initially and which AOIs are receiving attention later on.
Typically, the sequence is calculated as the average order that each AOI was vis-
ited. Keep in mind that many participants may not have experienced that exact
same order. Sequence is just a best estimate. We also recommend looking at a
binning chart (see Figure 7.8 ) as another view on sequence of AOIs.

TIME TO FIRST FIXATION
In some situations it's helpful to know how long it takes users to first notice a
particular element. For example, you may know that users spend only 7 seconds
on average on a page, but you want to make sure that a specific element, such as
a “continue” or ''sign up'' button, is noticed within the first 5 seconds. It's help-
ful that most eye-tracking systems time stamp each fixation (i.e., the exact time
that each fixation occurred).