Assessing cognitive load on web search is useful for characterizing search system features and search tasks with respect to their demands on the searcher’s mental effort. It is also helpful for examining how individual differences among searchers (e.g. cognitive abilities) affect the search process. We examined cognitive load from the perspective of primary and secondary task performance. A controlled web search study was conducted with 48 participants. The primary task performance components were found to be significantly related to both the objective and the subjective task difficulty. However, the relationship between objective and subjective task difficulty and the secondary task performance measures was weaker than expected. The results indicate that the dual-task approach needs to be used with caution. INTRODUCTION AND BACKGROUND Web search behavior is affected by the task, system, and individual searcher characteristics. Search tasks and their characterization have been a subject of recent systematic studies [1-3]. In particular, researchers have focused on the effects of task complexity and difficulty on information search process [4-10]. One kind of difficulty encountered by searchers is related to mental, or cognitive, requirements imposed by the search system or the task itself. Understanding factors that contribute to user’s cognitive load on search tasks is crucial to identifying search system features and search tasks types that impose increased levels of load on users. As new interactive features are introduced into the information search systems we need to understand what determines their acceptance and why some evidently useful functions are not widely used. For example, user relevance feedback is a feature that has been reported to be avoided by users due to the heightened cognitive load [11]. Among other factors affecting search performance are the user’s cognitive characteristics (e.g., [12, 13]). Methods used to date in assessing cognitive load included searcher observation, self-reports (e.g., using questionnaires, think-aloud protocols, and post-search interviews), dual-task techniques [14, 16], and various approaches that employ external devices to collect additional data on users (e.g., eye-tracking, pressure-sensitive mouse and other physiological sensors [17]). The latter two groups of techniques have the advantage of enabling real-time, ontask data collection. However, the use of external devices can be expensive and impractical. Hence, the promise of dual-task (DT) method that allows for an indirect objective assessment of mental effort on the primary task. Only few studies employed this method to assess cognitive load in online search tasks (e.g., [14, 15, 18]). The article discusses the dual-task method as the technique for assessing cognitive load on web search tasks and presents research that *Address correspondence to this author at the Department of Library & Information Science, School of Communication and Information, Rutgers University, New Brunswick, NJ, USA; E-mail: WebCog@gwizdka.com contributes to better understanding of how objective task difficulty affects searchers’ behavior and their perception of task difficulty.
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