Early prototyping of user interfaces is an established good practice in interactive system development. However, prototypes cover only some usage scenarios, and questions dealing with number of required steps, possible interaction paths or impact of possible user errors can be answered only for the specific scenarios and only after tedious manual inspection. We present a tool (MIGTool) that transforms models of the behavior of a user interface into a graph, upon which usage scenarios can be easily specified, and used by MIGTool to compute possible interaction paths. Metrics based on possible paths, with or without user navigation errors, can then be computed. For example, when analyzing four mail applications, we show that Gmail has 3 times more shortest routes, has twice more routes that include a single user error, has routes with 13þwer steps, but has also optimal routes with the smallest probability to be chosen. Without MIGTool, this kind of analysis could only be done after building some prototype of the system, and then only for specific scenarios by manually tracing user actions and relative changes to the screens. With MIGTool the exploration of suitability of a design with respect to different scenarios, or comparison of different design alternatives against a single scenario, can be done with just a partial specification of the user interface behavior. This is made possible by the ability to associate scenarios steps to required user actions as defined in the model, by an efficient strategy to identify complete execution traces that users can follow, and by computing a range of diverse metrics on these results.
