1 We propose a paradigm for ecologically valid, authentic, unobtrusive, automatic, data-rich, fast, robust, and sensitive evaluation of computer-assisted student performance. We instantiate this paradigm in the context of a Reading Tutor that listens to children read aloud, and helps them. We introduce inter-word latency as a simple prosodic measure of assisted reading performance. Finally, to validate the measure and analyze performance improvement, we report initial experimental results from the first extended in-school deployment of the Reading Tutor. Content areas: computer aided education, spoken language understanding, user interfaces, cognitive modeling, multimedia