Journal of Rehabilitation Medicine 51-3 | Page 47

J Rehabil Med 2019; 51: 193–200 ORIGINAL REPORT THE FUNCTIONAL INDEPENDENCE MEASURE 18-ITEM VERSION CAN BE REPORTED AS A UNIDIMENSIONAL INTERVAL-SCALED METRIC: INTERNAL CONSTRUCT VALIDITY REVISITED Roxanne MARITZ, MA 1,2 , Alan TENNANT, PhD 1,2 , Carolina FELLINGHAUER, PhD 1 , Gerold STUCKI, MD 1,2 and Birgit PRODINGER, PhD 1,2,3 , on behalf of the NRP74 StARS clinics 4 From the 1 Swiss Paraplegic Research, Nottwil, Switzerland, 2 Department of Health Sciences and Health Policy, University of Lucerne, Lucerne, Switzerland, 3 Faculty of Applied Health and Social Sciences, Technical University of Applied Sciences Rosenheim, Rosenheim, Germany and 4 NRP74 StARS clinics (aarReha Schinznach – Stefan Bützberger, Berner Klinik Montana – Dieter Ernst, Berner Reha Zentrum Heiligenschwendi – Jürg Wägli, Clinica di Riabilitazione EOC Novaggio & Faido – Giovanni Rabito, Clinica Hildebrand – Sandro Foiada, Clinique La Lignière – Nicolas Walther, Felix Platter Spital – Stefan Engelter, Hôpital du Valais Spital Wallis Centre Martigny, Sierre, Brig & Saint Amé – Els De Waele, Kantonsspital Baselland Bruderholz & Laufen – Beat Ritter, Klinik Schloss Mammern – Ruth Fleisch-Silvestri, Kliniken Valens Rehazentrum Valens, Rehazentrum Walenstadtberg & Rheinburg-Klinik – Stefan Bachmann, Rehaklinik Dussnang – Javier Blanco, Rehab Basel – Margret Hund – Georgiadis, Zürcher RehaZentrum Wald – Matthias Hermann, Rehaklinik Zihlschlacht – Michèle Bongetta, Spitäler Schaffhausen – Jan Kuchynka, Universitätsklinik Balgrist – Serge Altmann) Objective: Since the 1990s the Functional Indepen­ dence Measure (FIM™) was believed to measure 2 different constructs, represented by its motor and cognitive subscales. The practice of reporting FIM™ total scores, together with recent developments in the understanding of the influence of locally depen­ dent items on fit to the Rasch model, raises the ques­ tion of whether the FIM™ 18-item version can be re­ ported as a unidimensional interval-scaled metric. Design: Rasch analysis of the FIM™ using testlet approaches to accommodate local response depen­ dency. Patients: A calibration sample containing 946 cases of data from 11,103 patients undergoing neurologi­ cal or musculoskeletal rehabilitation in Switzerland in 2016. Results: Baseline analysis and the traditional testlet approach showed no fit with the Rasch model. When items were grouped into 2 testlets, fit to the Rasch model was achieved, indicating unidimensionality across all 18 items. A transformation table to con­ vert FIM™ raw ordinal scores to the corresponding Rasch interval scaled values was created. Conclusion: This study provides evidence that FIM™ total scores represent a unidimensional set of items, supporting their use in clinical practice and outcome reporting when applying the respective transforma­ tion table. This provides a basis for standardized re­ porting of functioning. Key words: outcome assessment (healthcare); psychome- trics; rehabilitation; activities of daily living; Rasch measure- ment model; Functional Independence Measure. Accepted Jan 24, 2019; Epub ahead of print Mar 5, 2019 J Rehabil Med 2019; 51: 193–200 Correspondence address: Roxanne Maritz, Rehabilitation Services & Care Unit, Swiss Paraplegic Research, 6207 Nottwil, Switzerland. E- mail: [email protected] T he primary outcome of rehabilitation is functioning (1). In order to document and monitor functio- ning, existing data collection tools can be used (2). LAY ABSTRACT The aim of this study was to look in detail at the FIM™, an assessment tool often used for patients undergoing rehabilitation. Some users report the FIM™ as 2 sco- res: one related to motor tasks, the other to cognitive tasks; others recommend reporting it as a single score including both motor and cognitive tasks. This study ex- plored whether it is statistically meaningful to sum all the points into a single FIM™ total score. The results support the current practice of summing the points into a single total score for patients undergoing musculo­ skeletal and neurological rehabilitation. The results also allowed an interval scale to be derived from the FIM™, enabling a broad range of calculations to be made using the FIM™ score, such as calculating the change in FIM™ outcomes from the time a patient is admitted to a reha- bilitation clinic until their discharge. The Functional Independence Measure (FIM™) is an assessment tool that is widely used in rehabilitation worldwide (3–6). The FIM™ is applied at the patient level to measure change throughout rehabilitation, at the institutional level to measure outcome quality, and at the national level for performance reporting or quality monitoring. Examples of use of the FIM™ are the reports of the Swiss National Association for Quality Development in Inpatient Care (ANQ) (7), the US model system for spinal cord injury (8) and traumatic brain injury (9), the Canadian Institute for Health Information (CIHI) (10), and the Australa- sian Rehabilitation Outcome Centre (AROC) (11). Furthermore, FIM™ measures can serve as a basis for inpatient rehabilitation payment (12). As with any assessment tool, in order to report valid total scores, certain psychometric standards must be met, including the assumption of unidimensionality. Furthermore, if an assessment tool is used to calculate change scores, it must be interval-scaled, rather than ordinal-scaled (6, 13). The Rasch measurement model can be used to examine assumptions such as unidimensionality or local item dependencies. Where satisfactory fit of data This is an open access article under the CC BY-NC license. www.medicaljournals.se/jrm Journal Compilation © 2019 Foundation of Rehabilitation Information. ISSN 1650-1977 doi: 10.2340/16501977-2525