9 Foundations for Designing and Evaluating User Interfaces Based on the Crossing Paradigm GEORG APITZ and FRANC ¸ OIS GUIMBRETI ` ERE HCIL, University of Maryland and SHUMIN ZHAI IBM Almaden Research Center Traditional graphical user interfaces have been designed with the desktop mouse in mind, a device well characterized by Fitts’ law. Yet in recent years, hand-held devices and tablet personal computers using a pen (or fingers) as the primary mean of interaction have become more and more popular. These new interaction modalities have pushed the traditional focus on pointing to its limit. In this paper we explore whether a different paradigm—goal crossing-based on pen strokes—may substitute or complement pointing as another fundamental interaction method. First we describe a study in which we establish that goal crossing is dependent on an index of difficulty analogous to Fitts’ law, and that in some settings, goal crossing completion time is shorter or comparable to pointing performance under the same index of difficulty. We then demonstrate the expressiveness of the crossing-based interaction paradigm by implementing CrossY, an application which only uses crossing for selecting commands. CrossY demonstrates that crossing-based interactions can be more expressive than the standard point and click approach. We also show how crossing-based interactions encourage the fluid composition of commands. Finally after observing that users’ performance could be influenced by the general direction of travel, we report on the results of a study characterizing this effect. These latter results led us to propose a general guideline for dialog box interaction. Together, these results provide the foundation for the design of effective crossing-based interactions. This work has been supported in part by Microsoft Research for the Microsoft Center for Interaction Design and Visualization at the University of Maryland and by NSF grant #0414699. Early partial results of this work had appeared as conference publications Accot and Zhai [2002] and Apitz and Guimbreti` ere [2004]. Authors’ addresses: G. Apitz, Human-Computer Interaction Lab, University of Maryland, A.V. Williams Building, College Park, MD, 20742; email: geapi@cs.umd.edu; F. Guimbreti` ere, Cor- nell University, Information Science, 301 College Avenue, Ithaca, NY, 14850; email: francois@ cs.cornell.edu; S. Zhai, IBM Almaden Research Center, 650 Harry Road, San Jose, CA, 95120; email: zhai@almaden.ibm.com. Permission to make digital or hard copies of part or all of this work for personal or classroom use is granted without fee provided that copies are not made or distributed for profit or commercial advantage and that copies show this notice on the first page or initial screen of a display along with the full citation. Copyrights for components of this work owned by others than ACM must be honored. Abstracting with credit is permitted. To copy otherwise, to republish, to post on servers, to redistribute to lists, or to use any component of this work in other works requires prior specific permission and/or a fee. Permissions may be requested from Publications Dept., ACM, Inc., 2 Penn Plaza, Suite 701, New York, NY 10121-0701 USA, fax +1 (212) 869-0481, or permissions@acm.org. C  2010 ACM 1073-0516/2010/05-ART9 $10.00 DOI 10.1145/1746259.1746263 http://doi.acm.org/10.1145/1746259.1746263 ACM Transactions on Computer-Human Interaction, Vol. 17, No. 2, Article 9, Publication date: May 2010.