Validation and Shortening Questionnaires Used in Surveys and Trials on Populations with Substance use Disorders: A Method using the Fatigue Severity Scale

Abstract

BackgroundPrecise and valid measurements of health outcomes and exposures among people with substance use disorders (SUDs) is essential to improve health services and health within this group. Unfortunately, many of the standardized questionnaires are validated on other populations and are often too comprehensive, and insufficiently adapted to the group. This may introduce limitations on several aspects that can be assessed but also biases due to research participation fatigue. New methods are needed to validate shortened and adapted questionnaires for this population. This study aims to present a method on how to shorten validated questionnaires and assure a construct validity when applied to SUD populations. MethodsWe used the data from a nested cohort with responses on a nine-item Fatigue Severity Scale (FSS-9), and Visual Analog Fatigue Scale (VAFS) collected from 655 people with SUD in Bergen and Stavanger, Norway, in the period 2016-2020. A total of 225 people filled out FSS-9 and VAFS at least twice. We defined baseline as the first measurement of FSS and VAFS when the measurements were sorted chronologically per participant. A three-step method was used for validation and shortening: Checking of internal consistency; longitudinal confirmatory factor analysis; linear mixed model analysis.ResultsThe internal consistency of FSS-9 was excellent with a Cronbach’s coefficient α on 0.94 at baseline and 0.92 at the second measurement. When shortening FSS-9 to a three-item FSS (FSS-3) (items 5-7), the Cronbach’s coefficient α was 0.87 at baseline and 0.85 at the second measurement. The internal consistency was not affected when VAFS was added to FSS-9 and FSS-3, respectively. The longitudinal confirmatory factor analysis model between baseline and second measurement showed a well-fitted model for FSS-3 (χ2 = 13.33, degree of freedom = 8, P = 0.101) with a correlation on r = 0.52, P < 0.001 between the measurements. The linear mixed model analysis showed equal linear changes at individual level for FSS-3 (slope: 0.00, P > 0.05) and FSS-9 (slope: 0.01, P > 0.05) between the measurements.ConclusionThe ten items could be shortened to a three-item version with excellent validity and reliability. This method could be useful for validating and shortening other questionnaires among patients with SUD and other populations.