Evaluation of the longitudinal item-response model
The adequacy of the longitudinal item-response model was evaluated in several ways, by comparing the model estimated or simulated data to the actual observations in the trial in terms of item scores and SoS, and over time:
i) To detect any major overall bias in the item-response model, the model estimated CCCs were compared to the distribution of the observations across entire severity range, for each score in each item. ii) To detect score-specific bias at the item level, the proportion of patients having that score was compared between simulated data and the observed data, for each score in each item. iii) To assess the model’s longitudinal predictivity, a visual predictive check was conducted to compare the time course of model-simulated and observed SoS values. For additional rigor, the longitudinal visual predictive checks were also conducted for model-simulated proportion of patients having each score for each item.