But What Does It All Mean? Understanding Eye-Tracking Results (Part 4)

Part IV:  What is a scan path?

I believe I referred to “scan paths” or “fixation patterns” in earlier entries.  Understanding scan paths are critical to interpreting eye movement results.

A scan path is a sequence of eye fixations made when someone samples a visual scene. Put simply, a scan path is the path our eyes follow when looking at an image.

One scan path image shows the sequence of fixations made by a single user.  Please note that individual fixation patterns are very noisy. However, if you look at a number of these traces, patterns can emerge. What we look for in individual scan paths are “orderly viewing patterns”.  In other words, we try to determine whether or not a person’s order of fixation follows a logical flow through a page.

At Eyetools, we produce scan path images which look like the image to the right.  A key to this can be found on our website. 

Qualitative analysis of scan path patterns is critical to any (usability targeted) eye-tracking study. Scan paths can be especially useful when presenting to design, marketing, and usability groups.  They very effectively illustrate how individual users are navigating the page.  However, when doing your basic analysis, experience and a working knowledge of eye movement/scene search literature will get you the most reliable read from these results.

There are many ways to discuss gaze traces.

Here are just a few examples of the imagery I've found works well when explaining traces:

• Think of view flow as a stream.  Are users flowing smoothly and unhindered through page content? Are things like ads or section breaks helping to guide the eye, or is the design placing a big boulder in the stream forcing users to detour around a section?

• Is there a "spaghetti effect"? Do many individual scan path images look as though someone dropped a handful of spaghetti on the page? (splat!) If users are searching a page at random, they are probably lost and aren't internalizing the message your page is trying to convey (at least not in the most efficient manner). 

• Does viewing suffer from a "pendulum effect"?  Do viewers use a page element as an anchor point and oscillate back and forth in multiple directions?  (Think Foucault's Pendulum) This pattern can result in decreased read-though of content, and has a number of other implications.  However, the severity of this problem really depends on the individual page content, design/client priorities, etc.

From this point on, any number of other metrics can be considered for data analysis.  The data you choose to examine more closely all depends on what you want to learn from your study.   

Continuing the Discussion: Methods and More

A note on software:

At Eyetools, we use in-house software to analyze our data. This has allowed us to develop tools that many commercial software systems don't readily provide. I'm not entirely familiar with the limitations of commercial and open source tools, but I assume that a large portion of stage 1 (heatmap analysis) and stage 2 (qualitative scan path analysis) is very common place.

But regardless of what kind of analysis algorithms you use (commercial, in-house, open source, etc.) it's important for the experimenter to have a solid understanding of eye movement terminology, experimental methodologies, and eye tracking algorithms.   After all, it helps to know (1) what you are actually measuring with an eyetracker, and (2) what the measurements mean, and (3) what the limitations of your hardware and software are. (Well, that's true in any field isn't it?)

A note on methodology:

Task instructions and participant preparation (priming) are important concerns in any behavioral or survey style study.  Eye-tracking is no different.  Rather than discuss methods at length, I’ll just pop in some pointers to other discussions and resources. 

•    [pdf] Granka, L and Rodden, K (2006). Incorporating Eyetracking into User Studies at Google. ACM Conference on Human Factors in Computing Systems.
•    [blog] Matteo Penzo's  Introduction to Eyetracking: Seeing Through Your Users’ Eyes
•    Suman, S and Bradburn, NM (1983). Asking Questions. San Francisco: Jossey-Bass.