It is generally accepted that the cognitive development for a wide range of students can be improved through adaptive instruction-learning environments optimized to suit individual needs (e.g., Cronbach, Am Psychol 12:671-684, 1957; Lee and Park, in Handbook of research for educational communications and technology, Taylor & Francis Group, Routledge, New York, pp 469-484, 2007; Shute and Zapata-Rivera, in Handbook of research for educational communications and technology, Taylor & Francis Group, Routledge, New York, pp 227-294, 2007). It is vital in adaptive instruction to diagnose an individual's learning progress in terms of cognitive changes in complex problem-solving contexts. This study conceptualized the levels of learning progress, associating the development of expertise in domain learning with the structural features of mental models. The theory of mental models accounts for how people conceptualize problem situations. That is, a mentally represented problem space is a structure including diverse relationships. It is necessary that assessment tools be adapted for the complex, dynamic structure of mental models so that the diagnostic, formative information become more precise. In short, the proposed stage-sequential model of learning progress was theoretically justified as being able to serve as a diagnostic model of learning progress.