CONCLUSION
In this work, longitudinal item-response analysis was applied to the data of MDS-UPDRS Part III from the PPMI study. It revealed insight on the relationship between the items of motor examinations and the underlying movement impairment and on the deterioration of the motor function over time. The most useful tests for differentiating symptom severity among patients were those for the left side of the body, and the least useful were the tremor tests. Simulations showed remarkable potential of about 50% sample size reduction by the item-response method, compared to the conventional sum-of-score method, for detecting a range of potential drug effects. We encourage the research community to further explore the full potential of this methodology.