In an eyetracking experiment comparing different clickability clues, weak and flat signifiers required more user effort than strong ones.
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We conducted a quantitative experiment using eyetracking equipment and a desktop computer. We recruited 71 general web-users to participate in the experiment. Each participant was presented with one version of the 9 sites and given the corresponding task for that page. As soon as participants saw the target UI element that they wanted to click to complete the task, they said “I found it” and stopped.

We tracked the eye movements of the participants as they were performing these tasks. We measured the number of fixations on each page, as well as the task time. (A fixation happens when the gaze lingers on a spot of interest on the page).

Both of these measures reflect user effort: the more fixations and time spent doing the task, the higher the processing effort, and the more difficult the task. In addition, we created heatmap visualizations by aggregating the areas that participants looked at the most on the pages.
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When we compared average number of fixations and average amount of time people spent looking at each page, we found that:

The average amount of time was significantly higher on the weak-signifier versions than the strong-signifier versions. On average participants spent 22% more time (i.e., slower task performance) looking at the pages with weak signifiers.
The average number of fixations was significantly higher on the weak-signifier versions than the strong-signifier versions. On average, people had 25% more fixations on the pages with weak signifiers.

(Both findings were significant by a paired t-test with sites as the random factor, p < 0.05.) This means that, when looking at a design with weak signifiers, users spent more time looking at the page, and they had to look at more elements on the page. Since this experiment used targeted findability tasks, more time and effort spent looking around the page are not good. These findings don’t mean that users were more “engaged” with the pages. Instead, they suggest that participants struggled to locate the element they wanted, or weren’t confident when they first saw it.

Source: Flat UI Elements Attract Less Attention and Cause Uncertainty