“…In few cases (e.g., [28,41,62,69]), the participants are also directly interviewed in a subjective manner to collect their impression at the moment of the user study without the rigidity of a pre-structured survey in order to "capture ground-truth data" [62]. While using the system, think-aloud feedback [3,5,38,52,53,67,73,75] or direct observation [38,62] are often used to acquire qualitative data. This type of feedback appears to be the preferred technique in user studies when there are usability experts evaluating a system or if a system is designed for an expert in the field.…”