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

Presented in part to Health Outcomes 2005: Making a Difference, August 17–18, 2005, Canberra, Australia, and the British Society for Rheumatology, May 17–18, 2006, Glasgow, Scotland.
https://doi.org/10.1016/j.apmr.2006.10.005Get rights and content

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 (χ162 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 (χ22 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 (χ122 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.

Section snippets

Foot Posture Index

The FPI was developed in response to a commonly expressed need for better foot measures9, 10, 11, 12, 13 due to the absence of a widely accepted or adequately validated method for quantifying variation in foot posture in the clinical setting.14 The FPI consists of a series of criterion-based observations that combine to provide a quantification of postural variation in 3 major regions of the foot (rearfoot, midfoot, forefoot) in the 3 cardinal body planes. The scoring system uses a 5-point

Data Capture

In the first analysis, we captured FPI data from 143 people (98 men, 45 women) with a range of foot types, comprising 2 groups of participants involved in a series of validation experiments.6 Participants were either patients with Charcot-Marie-Tooth disease (n=12; 7 men, 5 women; age range, 8–63y) or healthy adults participating in a prospective injury study (n=131; 91 men, 40 women; age range, 18–65y). Ethics approval was granted by the university review boards of the University of Western

The FPI-8

The initial overall item-trait interaction of the original FPI-8 indicated that some misfit to the Rasch model was apparent within the data (χ162 test=27.63, P=.03). The residual mean value for the items was .05, with an SD of 1.35. A significant item-trait interaction indicated that some items of foot positioning were not measuring in a consistent manner among discrete groups of people with differing foot postures. An analysis of the threshold ordering indicated that one of the items

Discussion

The FPI was designed to provide an objective measure of foot posture quickly and easily in a clinical setting. While the validation and reliability of the FPI has been described previously, the aim of the current study was to describe in detail the internal construct validity of the FPI and to explore the fit of the data to the latest statistical models. Our results indicated that while the original FPI-8 demonstrated good internal consistency and may be of some clinical use, the data did not

Conclusions

The original FPI-8 demonstrated limitations in its internal construct validity, with 2 of the items (Helbing’s sign, congruence of the lateral border of the foot) demonstrating significant mismatching to the model. The removal of these 2 items following the clinical validation phase has addressed these issues. The finalized FPI-6 demonstrated good internal construct validity including good individual item fit and good overall fit to the model, along with a lack of DIF. The FPI-6 raw scores can

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    Copyright © 2007 American Congress of Rehabilitation Medicine and the American Academy of Physical Medicine and Rehabilitation. Published by Elsevier Inc. All rights reserved.