Systems combining the real and the virtual are becoming more and more prevalent. The Augmented Reality (AR) paradigm illustrates this trend. In comparison with traditional interactive systems, such AR systems involve real entities and virtual ones. And the duality of the two types of entities involved in the interaction has to be studied during the design. We therefore present the ASUR notation: The ASUR description of a system adopts a task-centered point of view and highlights the links between the real world and the virtual world. Based on the characteristics of the ASUR components and relations, predictive usability analysis can be performed by considering the ergonomic property of consistency. We illustrate this analysis on the redesign of a computer assisted surgical application, CASPER.