This paper describes some lessons learned from international adult numeracy assessments that can help in understanding the challenges that people, including both adults and school students, have when solving numeracy tasks and their levels of performance on functional mathematical problems. The paper presents a theoretical schema of five factors that predict, separately and in interaction, the complexity or level of difficulty of mathematically-related assessment tasks, including tasks that incorporate texts and require literacy or reading skills, which are very common in adults\u27 lives. The model was originally developed as part of the development of the Adult Literacy and LifeSkills survey in the mid \u2790s, but later adapted and effectively used within the Programme for the International Assessment of Adult Competencies (PIAAC), a.k.a. OECD Survey of Adult Skills. The five complexity factors described in the model are grouped into two factors addressing mainly textual aspects of tasks, and three factors addressing the mathematical aspects of tasks. These factors can assist test developers, researchers and educators in predicting task difficulty and in targeting the development of items and tasks to more efficiently cover the range of student performance and skill levels
