Jun Xiao, Richard Catrambone, John T. Stasko. Be Quiet? Evaluating Proactive and Reactive User Interface Assistants. In Matthias Rauterberg, Marino Menozzi, Janet Wesson, editors, Human-Computer Interaction INTERACT 03: IFIP TC13 International Conference on Human-Computer Interaction, 1st-5th September 2003, Zurich, Switzerland. IOS Press, 2003.