Structure of the Observed Learning Outcome (SOLO) science superitems were examined from the perspectives of Guttman Scaling (deterministic) and Item Response Theory (probabilistic). Differences between the measurement bases for the two approaches, and the results for a small case study, are reported. In general, the analyses confirmed the hypothesised patterns of learners' responses, but also revealed some discrepancies in particular superitems that can be interpreted within the framework of the SOLO taxonomy.