Evidence-Based Guidelines for Using the Short Form 36 in Cervical Dystonia Stefan J. Cano, PhD, 1,2 Alan J. Thompson, MD, 1 Khailash Bhatia, MD, 3 Ray Fitzpatrick, PhD, 4 Thomas T. Warner, PhD, 5 and Jeremy C. Hobart, PhD 1,2 * 1 Neurological Outcomes Measures Unit, Institute of Neurology, University College London, United Kingdom; 2 Department of Clinical Neurosciences, Peninsula Medical School, Plymouth, United Kingdom; 3 Sobell Department of Motor Neuroscience and Movement Disorders, University College London, United Kingdom; 4 Department of Public Health, University of Oxford, Oxford, United Kingdom; 5 Department of Clinical Neurosciences, Royal Free & University College Medical School, London, United Kingdom Abstract: We aimed to provide evidence-based guidelines for using the Short Form 36 (SF-36) as an outcome measure in cervical dystonia (CD). To do this, we tested the hypoth- esized relationships between items, scales, and summary measures of the SF-36 using psychometric analyses in data from a postal survey of 235 people with CD. Although the majority of subscales performed adequately, the Role Phys- ical and Role Emotional subscales had substantial floor and/or ceiling effects. Evidence did not support computing SF-36 Physical and Mental Component Summary scores. We propose guidelines that include the recommendation that these subscale and summary scores should be reported with caution. © 2006 Movement Disorder Society Key words: cervical dystonia; rating scale; reliability; valid- ity; health measurement; quality of life; psychometrics; SF-36 Patient rating scales are increasingly being used to evaluate the impact of cervical dystonia (CD). 1 The general recommendation is that both generic (i.e., appli- cable across many diseases) and disease-specific (e.g., specifically developed for CD) measures should be used. 2 Psychometric evidence must also be available (i.e., reliability, validity, and responsiveness). 3 Although this is emerging for disease-specific measures, 4 generic scales continue to be used based on the often untested assumption that they satisfy psychometric requirements across different disease groups. The Medical Outcomes Study 36-item Short Form Health Survey (SF-36) is the most widely used generic measure of health status 5 and has been used in dystonia. 6–8 However, recent studies have raised concerns about its use in neurological disorders. 9 –11 As such, this study aims to provide evidence-based guidelines for clinicians wishing to use the SF-36 as an outcome measure in CD to ensure that any inferences made are as valid as possible. PATIENTS AND METHODS Recruitment, Sample, and Data Collection A sample of 235 people with CD (selected using a random number generator) was recruited from a com- plete list of 1,110 members from the Dystonia Society of Great Britain. A booklet of questionnaires, including the SF-36, was administered as a postal survey following standard techniques to ensure a high response rate. The sampling and data collection strategies are described elsewhere. 4 Measurement Model The SF-36 measurement model (i.e., how the authors propose items in the SF-36 should be put into subscales, and how subscales are scored) groups the 35 of the 36 items into eight multi-item subscales: Physical Functioning (10 items), Role Limitations Physical (4 items), Bodily Pain (2 items), General Health Perceptions (5 items), Energy/Vitality (4 items), Social Functioning (2 items), Role Limitations Emotional (3 items), and Mental Health (5 items). Subscale scores are produced by simply summing the numbers cor- responding to each item response. This is known as Likert’s method of summated ratings. 12 For easier interpretation, scores are transformed to have a range of 0 (worst health) to 100 (best health). 5 In addition, the authors recommend the computation of two summary scores: a physical health score (Physical Component Score; PCS) and a mental health score (Men- tal Component Score; MCS). These are produced based on an algorithm using U.S. general population data and have a mean of 50 and a standard deviation of 10. The advantage of summary scores over the eight SF-36 sub- scale scores in clinical trials has been discussed previ- ously 10,13 and includes more easily interpretable scores that have better measurement precision and superior re- sponsiveness. The process recommended for generating SF-36 subscale and summary scores are assumptions that must be tested. Analyses The SF-36 measurement model makes three funda- mental assumptions that should be tested to determine *Correspondence to: Dr. Jeremy C. Hobart, Department of Clinical Neurosciences, Peninsula Medical School, Tamar Science Park, Ply- mouth, PL6 8DH, UK. E-mail: jeremy.hobart@pms.ac.uk Received 9 February 2006; Revised 6 July 2006; Accepted 15 July 2006 Published online 17 November 2006 in Wiley InterScience (www. interscience.wiley.com). DOI: 10.1002/mds.21187 122 S.J. CANO ET AL. Movement Disorders, Vol. 22, No. 1, 2007