BACKGROUND: The 9-item, Fatigue Severity Scale (FSS-9) has been widely used as an outcome measure in multiple sclerosis (MS). Modern psychometric theory, in the form of the Rasch measurement model, has set new quality standards for outcome measures by appraising a broad range of measurement properties in addition to the reliability and validity emphasized by classical test theory. OBJECTIVE: To appraise the FSS-9 by application of the Rasch model. METHOD: The FSS-9 was posted to patients with clinically definite MS in two centers in the United Kingdom. Analysis was based on 416 records (55% response). RESULTS: The 9-item scale failed to meet Rasch model expectations. Two items had poor discrimination across the scale, and two further items showed bias for factors such as age. Removal of these four items provided a valid 5-item Rasch scale that satisfied strict tests of unidimensionality. CONCLUSION: Summating the nine items of the FSS-9 is invalid. Five items (FSS-5), which seem to be measuring the social impact of fatigue, provide a strictly unidimensional Rasch scale. Studies using the FSS-9 may need to be re-evaluated using the FSS-5, preferably using the Rasch transformed scores.
BACKGROUND: The 9-item, Fatigue Severity Scale (FSS-9) has been widely used as an outcome measure in multiple sclerosis (MS). Modern psychometric theory, in the form of the Rasch measurement model, has set new quality standards for outcome measures by appraising a broad range of measurement properties in addition to the reliability and validity emphasized by classical test theory. OBJECTIVE: To appraise the FSS-9 by application of the Rasch model. METHOD: The FSS-9 was posted to patients with clinically definite MS in two centers in the United Kingdom. Analysis was based on 416 records (55% response). RESULTS: The 9-item scale failed to meet Rasch model expectations. Two items had poor discrimination across the scale, and two further items showed bias for factors such as age. Removal of these four items provided a valid 5-item Rasch scale that satisfied strict tests of unidimensionality. CONCLUSION: Summating the nine items of the FSS-9 is invalid. Five items (FSS-5), which seem to be measuring the social impact of fatigue, provide a strictly unidimensional Rasch scale. Studies using the FSS-9 may need to be re-evaluated using the FSS-5, preferably using the Rasch transformed scores.
Authors: Anners Lerdal; Anders Kottorp; Caryl Gay; Bradley E Aouizerat; Carmen J Portillo; Kathryn A Lee Journal: Qual Life Res Date: 2011-03-10 Impact factor: 4.147
Authors: Nadina B Lincoln; Lucy E Bradshaw; Cris S Constantinescu; Florence Day; Avril Er Drummond; Deborah Fitzsimmons; Shaun Harris; Alan A Montgomery; Roshan das Nair Journal: Health Technol Assess Date: 2020-01 Impact factor: 4.014
Authors: Thiago H Freitas; Elias Andreoulakis; Gilberto S Alves; Hesley L L Miranda; Lúcia L B C Braga; Thomas Hyphantis; André F Carvalho Journal: World J Gastroenterol Date: 2015-06-07 Impact factor: 5.742
Authors: Arthur Derksen; Lidwine B Mokkink; Marc B Rietberg; Dirk L Knol; Raymond W J G Ostelo; Bernard M J Uitdehaag Journal: Qual Life Res Date: 2013-02-26 Impact factor: 4.147