Menke, James Michael DC, PhD
SPINE: November 29, 2021 – Volume – Issue –
doi: 10.1097/BRS.0000000000004298
Abstract
Study Design.
The 2001 Functional Rating Index (FRI) was not developed under today’s standard psychometric analysis. The original data of 108 cases, were re-analyzed using Rasch item response theory. In addition, two alternative forms were administered to an additional 140 patients for comparison.
Objective.
The FRI was never developed to standards as set by the FDA in 2009. Even so, the FRI has been cited over 300 times, translated into multiple languages, used as primary clinical and research outcomes, and used to calibrate concurrent, construct, and content validity for other surveys. This study tests the FRI using the modern item response theory.
Summary of background data.
The 2001 FRI data showed internal agreement among items and weak item-total correlation items. The FRI’s true reliability and validity have never been confirmed.
Methods.
The original 2001 FRI 108 and two new versions with 140 respondents with back pain were compared with Rasch analysis characteristics of unidimensionality, local independence, monotonicity, and differential item functioning.
Results.
All three versions measured more than one latent construct. Form 2001 had two items that were non-monotonic, four with differential item functioning (DIF), five items with poor infit, and four with poor outfit. Form A had five non-monotonic items, five had DIF, three had poor infit, and three had poor outfit. Form B had no non-monotonic items, one item with DIF, three items with poor infit, and two items with poor outfit.
Conclusion.
The original FRI and alternative forms fail crucial psychometric tests and assesses more than one latent construct. It is thus unfit as a pain, function, and disability assessment. Only reducing the number of Likert choices improved the test. Other back pain assessments should be used instead, and all surveys would benefit from periodic item responses to adjust to shifts in grammar and meaning.
Level of Evidence: 3
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