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.