Item-level correlations with anchors
To evaluate the properties of the individual items, the relationships with anchor measures was explored. Specifically, correlations with the PGIS anchor were explored, and correlations were calculated using data collected at Visit 2, where the PRO data used was from the closest assessment to Visit 2 (provided this was within four days) in the psychometric analysis population. For item 1–3, the point-biserial correlation coefficient was determined due to the use of a dichotomous scale’. For item 4–6, the polyserial correlation coefficient was determined for these severity items. For all correlation coefficients, the following interpretation cut-offs were applied: ‘weak correlation’: 