R-Code to analyze three pain items (movement pain, worst pain, least pain)

To assess postoperative pain intensity in adults the Numeric Rating Scale (NRS) is used. This scale has shown acceptable psychometric features, though its scale properties need further examination. We aimed to evaluate scale properties of the NRS using an item response theory (IRT) approach. Data from an international postoperative pain registry (QUIPS) was analyzed retrospectively. 346.892 adult patients (age groups: 18-20: 1.6%, 21-30: 6.7%, 31-40: 8.3%, 41-50: 13.2%, 51-60: 17.1%, 61-70: 17.3%, 71-80: 16.4%, 81-90: 3.9%, >90: 0.2%) were included. 55.7 % are female and 38% had preoperative pain. Three pain items (movement pain, worst pain, least pain) were analyzed using four different IRT-models: partial credit model (PCM), generalized partial credit model (GPCM), rating scale model (RSM) and graded response model (GRM). Fit indices were compared to decide which model fits best (lower fit indices indicate a better model fit). Subgroup analyses were done for sex and age groups.