Title of Document: USERS ’ SPATIAL ABILITIES AFFECT INTERFACE USABILITY OUTCOMES

The usability of a system depends both on inherent characteristics of the system and on its users. This paper argues that a major source of differences among users is variations in spatial ability, but that variations in different types of spatial ability affect components of usability differently. In two experiments, I investigated a simple model of the relationships of spatial visualization ability, spatial orientation ability, and spatial working memory with the usability constructs of efficiency and effectiveness. Both experiments used Wikipedia search as a representative information search task. The first experiment used a desktop computer interface, and the second experiment used a pair of mobile devices with widely different screen sizes. Better spatial orientation ability corresponded to faster performance on efficiency across devices. Better spatial visualization ability corresponded to slower performance on larger screens, but faster performance on smaller screens. Better spatial visualization ability also predicted better effectiveness in both experiments. These results suggest that spatial ability is a useful way to characterize users and to improve usability testing, and that its effects vary in systematic ways depending on characteristics of the tested interface and on which metrics are chosen