There's More to Direct Manipulation than meets the Eye

Abstract

The term “direct manipulation” (or DM) often evokes images of interfaces which are intuitive, obvious, and easy to learn. We conducted an experiment to determine whether subjects could learn to use a DM interface without instruction, i.e., whether they could learn the interface syntax on their own merely by inspection and exploration of the interface. The research vehicle was a prototype DM applicationdesigned to allow end users to customize a telecommunications application. Three variations of the interface were created by manipulating elements of the DM syntax, specifically, moded operations and rules about selectingobjects before acting on them. Subjects carried out a set of five tasks in the presence of an experimenter, who was allowed to provide structured help when the subject could not make further progress. Results indicated that the syntax manipulations affected both the number and type of user errors and the amount of help needed to complete the tasks: the use of modes and selection rules significantly interfered with learning, and only four subjects out of thirty were able to perform the complete set of tasks without experimenter assistance. We also found, however, that more than half of the errors made by subjects were not directly related to syntax manipulations. These errors appear to stem more from conceptual problems, i.e., mismatches between the user's developing model of the interface and the model instantiated by the interface designer in the rules of interaction. These conceptual problems were observed across syntax manipulations and represent a significant portion of user's difficulties in learning the interface. Thus, our results shed light on the relationship between interface syntax, learning and usability in the DM paradigm, but they also point out the need for a cognitive account of the processes by which users acquire knowledge of interface characteristics and how that knowledge is related to interface design elements.