C. Baranauskas et al. (Eds.): INTERACT 2007, LNCS 4663, Part II, pp. 653 – 656, 2007. © IFIP International Federation for Information Processing 2007 A New Performance Measure Taking into Account the Mental Load in Mobile Text Entry Tasks Franck Poirier and Hamed H. Sad Université de Bretagne-Sud VALORIA - Centre de recherche – 56000 Vannes-France franck.poirier, sad@univ-ubs.fr Abstract. Text entry research has received a lot of attention in recent years because of the need for more effective and usable entry methods on mobile devices. Technical limitations such as screen size have led to the design of entry interfaces that mentally load the user in order to obtain better performances. Current evaluation methodologies of these interfaces focus on text entry speed and error rate but don’t pay enough attention to the mental load. In this paper, we concentrate on the evaluation of the load’s effect on text entry process and we present a comparative evaluation of three mobile text entry methods with and without the application of a secondary task. We also define a performance measure that takes into account the mental load characteristic for a given text entry interface. Keywords: Mobile text entry, mental load, evaluation, secondary task. 1 Introduction Mobile text entry process can be viewed as the integration of two sub-processes. The first is the text creation in which an idea should be translated into words and sentences in a particular language. The second is the process of converting these words and sentences to an electronic form using a text entry technique with minimal resources (small screen, tiny soft keyboard...). In mobile context, these two sub-processes are always executed concurrently, which causes an increase in the mental load compared to the desktop interaction context. Moreover, in the mobile environment, the user may be on the move while executing the two processes, which also increases the load. In the literature of human-computer interaction (HCI) many words and expressions have been used to describe “How busy is the operator?”. In interface design, the term “cognitive load” is used. But, in this domain, this word is more general and indicates one or more of the following three components: perceptual load, central processing and motor load [3]. In this paper, we use the term “mental load” to indicate the amount of resources used for the perceptual and central processing activities, where memory operations and decision-making are involved. 2 Mental Load in Mobile Text Entry A review of mobile text entry methods shows that, for their evaluation, the motor