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 (X-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 (X-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 (X-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.