Scaling the sickness impact profile using item response theory: an exploration of linearity, adaptive use, and...

The objective of the study was to enhance the clinical interpretation and practicality of the widely used comprehensive Sickness Impact Profile.

Item Response Theory (extension of the Rasch model) was used to calibrate the severity of the SIP items, to assess item bias and to construct equally severe short forms of the SIP that can be used interchangeably. The scores of 1507 subjects were analyzed.

Of the 127 SIP items, 82 items fitted the extended Rasch model, i.e., the observed proportions of sickness level groups endorsing the items corresponded to the proportions expected by the model. The item severity hierarchy allowed a more straightforward interpretation of the calibrated SIP-82 scores. Some items showed bias in age, gender, or diagnosis groups. The equivalent short forms agreed sufficiently well with the calibrated SIP-82 item pool to be used interchangeably. We observed a moderate correlation between the original SIP item severity weights and the Rasch item severity calibrations (r=0.53).

The interpretability and practicality of the SIP was enhanced by the IRT calibration. Using the item calibrations, short forms can be assembled that can be used interchangeably.