The influence of users functional state on the usability evaluation
In recent years, the usability became a central issue for program interfaces design. This study determined the influence of user’s functional states on graphical user interface usability evaluation. We hypothesized the strong link between changes in psychophysiological indicators of users’ functional states and the way they evaluate the interface usability. 26 male students of technical specialties (mean age – 21 years) operated two users interfaces (hierarchical menus). Menus consisted of information of objects and the task of participants was to classify these objects using six categories. In addition to such performance measures as task completion time and task completion rate several psychophysiological parameters were recorded – electrocardiogram, galvanic skin response, electroencephalogram, respiratory rate. For each participant the experiment consisted of two blocks during which subjects performed the task of classification in menus. Blocks contained 30 objects to classification. Between these blocks, there was a relaxation break to change functional states of the subjects using relaxation procedure (sensory reproduction). We also used the System Usability Scale (SUS) to evaluate the subjective usability level. The regression analysis showed the importance of the functional state dynamic for the construction of the model (R2=0,667, p<0,001) considering two significant determinants of users subjective system usability evaluation – eeg alpha rhythm power and heart rate variability. Furthermore, our finding revealed that functional state dynamic indirectly influence on usability through interface satisfaction. Differences in psychophysiological parameters point to the need to consider more strongly the factor of functional state in usability research and practice.