Four hundred and seventy-three patients with ankylosing spondylitis received placebo or active non-steroidal anti-inflammatory drug (NSAID) for 6 weeks, and changes between entry and completion were captured by BASDAI and the individual components. Principle component analysis (PCA) was used to explore the best combinations of variables in decreasing order of explained total dispersion and to assess whether a single sum (or algebraic expression) best defined disease activity status.