The structure of observed learning outcomes (SOLO) taxonomy is a model that describes levels of increasing complexity in students' understanding of subjects. It was proposed by John B. Biggs and Kevin F. Collis.^[1]

The model consists of five levels of understanding:^[2]

• Pre-structural – The task is not attacked appropriately; the student hasn't really understood the point and uses too simple a way of going about it. Students in the pre-structural stage of understanding usually respond to questions with irrelevant comments.
• Uni-structural – The student's response only focuses on one relevant aspect. Students in the uni-structural stage of understanding usually give slightly relevant but vague answers that lack depth.
• Multi-structural – The student's response focuses on several relevant aspects but they are treated independently and additively. Assessment of this level is primarily quantitative. Students in the multi-structural stage may know the concept in tidbits but don't know how to present or explain it.
• Relational – The different aspects have become integrated into a coherent whole. This level is what is normally meant by an adequate understanding of some topic. At the relational stage, students can identify various patterns & view a topic from distinct perspectives.
• Extended abstract – The previous integrated whole may be conceptualised at a higher level of abstraction and generalised to a new topic or area. At this stage, students may apply the classroom concepts in real life.