Lynne Cooke made studies on concurrent think-aloud protocol and assessed how well verbal reports correlate with eye movements in usability testing. She used eye-tracking equipment to compare the contents of verbalization and eye movement during task performance, and to study what eye movements reveal when users fall into silence or use verbal fillers without sensible contents. The participants of the studies included five women and five men and the studied system was a website with lots of textual information.
The results of her studies showed that 80% of the task performance time users’ eye movements verified their verbal reports. The test users used 58% of their verbalization units in reading text off the screen, 19% describing their current or future procedure, 10% in observative comments, and only 5% in explanatory descriptions. The eye movements also revealed that the users continued the performance of given tasks also when they fell silent or used verbal fillers. Nevertheless, silences were signs of cognitive-processing difficulties, and verbal fillers enabled users to visually explore and mentally process information during an on-screen search.