The Foot Posture Index: Rasch analysis of a novel, foot-specific outcome measure

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Arch Phys Med Rehabil. 2007 Jan;88(1):88-93.

The Foot Posture Index: Rasch analysis of a novel, foot-specific

outcome measure.

Keenan AM, Redmond AC, Horton M, Conaghan PG, Tennant A.

Academic Unit of Musculoskeletal Disease, University of Leeds,

Leeds, UK.

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 (chi(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 (chi(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 (chi(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.