Eye Tracking vs. Mouse Tracking

When explaining GazeHawk to people we often hear comparisons to “mouse tracking” services that track where the mouse moves on a page. It’s a reasonable comparison: both technologies build heatmaps as a main visualization. However, just because they use the same visualization doesn’t mean they are interchangeable. Given that there are a number of articles touting the similarities between mouse tracking and eye tracking, it worth publishing an analysis of the differences.

One such article recently posted by ClickTale claims that mouse movement is highly correlated to eye movement and can be used to draw similar conclusions. There are a few claims in that article that just aren’t supported by the evidence, though, and a few that may no longer be true thanks to recent advances in eye-tracking technology.

Eye Tracking/Mouse Tracking Correlation

The post argues that there is an 84%-88% correlation between eye and mouse movement, citing a Carnegie Mellon paper from 2001. This is a very questionable interpretation of the paper’s claims:

First, the paper measures areas on the screen where both the eye looked and the mouse moved at some point, not necessarily at the same time. The exact quote:

Of the regions that a mouse cursor visited, 84% of them were also visited by an eye gaze. Furthermore, among the regions that the eye gaze didn’t visit, 88% of them were not visited by the mouse cursor, either.

This has no temporal quality to it. All it says is that 84% of the time, if a user’s mouse moved over a given region of the screen, at some point the user’s eyes also crossed over that same region (and the contrapositive happens 88% of the time). One of the questions eye tracking is really good at answering is “what’s the most eye-catching thing on my page”. Knowing where the user’s mouse went at some point doesn’t really answer that question.

Second, they also cite 2 other papers doing similar research that quote lower numbers (they neglect to mention this data and use the papers to support other points):

• 69% in a paper by Lynne Cooke
• 42% in a paper by Google (page 29)

Finally, ten seconds worth of Googling also pulls up a more recent paper that paints a much grimmer (though anecdotal) picture. Even when only looking at the Y-axis (the more common direction for the mouse to follow the eye), a mere 56 of 175 Google result pages had mouse movement mimic eye movement. That’s only 32%.

Cons of Eye Tracking

The post also mentioned 3 cons for eye tracking (cost, scope, intrusive). These are completely valid for traditional eye tracking hardware: it’s expensive and time consuming. In fact, those three problems are all reasons why we started GazeHawk in the first place. Given the introduction of GazeHawk’s technology, the three cons are significantly less relevant:

• Cost – I think our pricing speaks for itself. We bring the cost of eye tracking down to $49/user.
• Limited Scope – When using custom hardware to run an eye tracking study, you are forced to use a certain monitor size, as well as a smaller sample size. GazeHawk is not limited by either of these, since we use users’ own computers/webcams and have the users test themselves with a self-moderated study.
• Strong Observer Effect – The Hawthorne effect is the problem of subjects changing their actions because they’re aware that they’re being tracked. When you bring a user into a lab you run into this problem. We’ve explicitly heard from a number of testers that when trying out the software they completely forget that they’re being tracked. The only difference between viewing a page and viewing a page during a GazeHawk study is that you have to calibrate (follow a dot around the screen with your eyes) before starting, and there’s a little light on your webcam telling you it’s on. This is not as passive as tracking a user without their knowledge, but it’s pretty close.

Conclusion

In the end the big question is “should I use eye tracking or mouse tracking?” Mouse tracking does have the advantage of being passive: you can silently track all of your users, while eye tracking requires running an opt-in study. While we’re obviously not objective, I would argue that eye tracking gives you information that cannot be inferred from mouse data alone. Of course, you have to factor in costs and budgets, but as these methods become cheaper and easier it becomes more viable to employ more than one of them.

Eye tracking is not the holy grail of usability studies, but it’s definitely a unique and valuable tool to have in your UX/usability/conversion rate optimization arsenal.

Thanks to my cofounder, Joe, for proofreading, and Max Hutchinson for helping with research.