Local independence
PROM
SEPARATION
Regarding local independence, residual correlations were above 0.2 in five cases, however, above 0.3 in no case. Correlations and corresponding items are shown in Table Largest standardized residual correlationsIn terms of reliability, the HidroQoL achieved a PSI of 0.85 and showed a person separation of 2.36, meaning that the PROM was able to differentiate between at least two statistically significantly different severity groups. Item separation had a value of 12.99 with a high item reliability of 0.99, reflecting almost 13 levels of item difficulty in the data. Thus, the difficulty hierarchy of the items could be verified as an indicator of the construct validity of the instrument.