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.