OBJECTIVES: To apply item response theory as a framework for studying measurement error in superiority trials which use patient-reported outcome measures (PROMs). METHODS: We reanalyzed data from the The Total or Partial Knee Arthroplasty Trial, which compared the Oxford Knee Score (OKS) responses of patients undergoing partial or total knee replacement, using traditional sum-scoring, after accounting for OKS item characteristics with expected a posteriori (EAP) scoring, and after accounting for individual-level measurement error with plausible value imputation (PVI). We compared the marginalized mean scores of each group at baseline, 2 months, and yearly for 5 years. We used registry data to estimate the minimal important difference (MID) of OKS scores with sum-scoring and EAP scoring. RESULTS: With sum-scoring, we found statistically significant differences in mean OKS score at 2 months (P = 0.030) and 1 year (P = 0.030). EAP scores produced slightly different results, with statistically significant differences at 1 year (P = 0.041) and 3 years (P = 0.043). With PVI, there were no statistically significant differences. CONCLUSION: Psychometric sensitivity analyses can be readily performed for superiority trials using PROMs and may aid the interpretation of results.