The Foot Posture Index: Rasch Analysis of a Novel, Foot-Specific Outcome Measure

Abstract

Keenan A-M, Redmond AC, Horton M, Conaghan PG, Tennant A. The Foot Posture Index: Rasch analysis of a novel, foot-specific outcome measure. Objective To investigate the internal construct validity of a clinician-assessed measure of foot position, the Foot Posture Index (FPI), versions FPI-8 and FPI-6. Design Rasch analysis of baseline FPI scores from studies conducted during the development of the instrument. Setting A community-based and a hospital-based study, conducted at 2 institutions. Participants Measures were obtained from 143 participants (98 men, 45 women; age range, 8–65y). Interventions Not applicable. Main Outcome Measures Rasch analysis was undertaken using RUMM2020 software in order to evaluate the following properties of the FPI: unidimensionality of each item included in the FPI, the differential item functioning (DIF) of each item, and item and person separation indices. Results In the developmental draft of the instrument, the 8-item FPI-8 showed some misfit to the Rasch model (χ 16 2 test=27.63, P=.03), indicating lack of unidimensionality. Two items were identified as problematic in the Rasch modeling: Achilles’ tendon insertion (Helbing’s sign), which showed illogical response ordering and “congruence of the lateral border of the foot,” which showed misfit, indicating that this item may be measuring a different construct (χ 2 2 test=15.35, P<.01). All FPI-8 items showed an absence of DIF, and the person separation index (PSI) was good (PSI=.88). The revised FPI-6, which does not include the 2 problematic items, showed unidimensionality (χ 12 2 test=11.49, P=.49), indicating a good overall fit to the model, and improvement over the preliminary version. With the removal of the 2 problematic items, there were no disordered thresholds; all items remained DIF free and all individual items displayed a good fit to the model. The person-separation index for the FPI was similar for both the 8-item (FPI-8=.880) and 6-item (FPI-6=.884) versions. Conclusions The original FPI-8 showed significant mismatching to the model. The 2 items in the FPI-8 that were identified as problematic in clinical validation studies were also found to be contributing to the lack of fit to the Rasch model. The finalized 6-item instrument showed good metric properties, including good individual item fit and good overall fit to the model, along with a lack of differential item functioning. This analysis provides further evidence for the validity of the FPI-6 as a clinical instrument for use in screening studies and shows that it has the potential to be analyzed using parametric strategies.