Editorial In a time of increasing use of outcome measures in clinical practice, quality control and audit procedures, physiatrists need to acquire the necessary expertise to be able to select the appropriate tools, administer them thoughtfully, and interpret correctly the results (1) . An outcome measure is essentially an evaluative tool for assessing the magnitude of some longitudinal change (in impairment, functioning, activities, participation, etc.) in an individual or group (2) ; in Physical and Rehabilitation Medicine what is subject to change often is a ‘latent trait’, ‘trait’ meaning a hypothetical construct, domain, ability or other (e.g. functional independence, manual dexterity, locomotor capability) and ‘latent’ meaning that it cannot be measured directly but is ‘hidden’ within the person, who may manifest it through a set of behaviours assessed by a series of questions (items) (3) . In order to be useful for their intended purposes, the rating scales and questionnaires measuring ‘latent traits’ must provide information that allows valid inferences and decisions to be made. Basic classical test theory is still widely used in peer-reviewed, indexed journals for validating these tools, in both original and translated versions. These papers are based mainly on analysis of internal consistency [using Cronbach’s alpha, well known for its limits (4) ], reproducibility, and criterion-related validity (usually the demonstration of a moderate to good correlation with some other measure of the trait under study). This is a superficial approach that neglects standard criteria and practical attributes that need to be considered when evaluating the psychometric properties of a measurement tool (5-11) , and it does not provide information about many essential psychometric characteristics, such as the evaluation of how well an item performs in terms of its relevance or usefulness for measuring the underlying construct, the amount of the construct targeted by each question, the possible redundancy of the item relative to other items in the scale, and the appropriateness of the response categories (12) . Furthermore, the caveats emerging from the use of modern (e.g. Rasch) measurement methods are often neglected or disregarded, probably due to a lack of familiarity with these methods and their results. As an example, some years ago the measurement properties of the Lequesne index of severity for osteoarthritis of the hip in elderly people were examined and major limitations were found with the convergent validity and the unidimensional structure of the measure (13) . This paper was cited also by a review on assessment of disability associated with osteoarthritis, commenting that this finding adds to the literature indicating problems with this measure (14) , but the Lequesne index continues to be a favourite amongst clinicians, and several recent papers in PubMed include it as an outcome measure. The same applies for the Berg Balance Scale, a measure with a rating scale structure needing refinements (15) , but still used in its original version by dozens of recent studies. In addition, papers reporting the validation of a scale in different languages give little insight to readers if detailed methods of cross-cultural adaptation and validation are not applied (16-18) . In the last few years our group has published a number of papers reporting psychometric analyses − using both classical test theory and Rasch analysis − of outcome measures to investigate a wide range of metric characteristics (19-29) . The purpose of this paper is to summarize some basic results of these studies, in order to provide insights for selecting and/or revising outcome measures in Physical and Rehabilitation Medicine (PRM). We focus our I. Editorial 5 Vol 19 I Nº 2 I Ano 18 (2010) I Revista da Sociedade Portuguesa de Medicina Física e de Reabilitação Practical lessons learned from use of rasch analysis in the assessment of outcome measures Franco Franchignoni (1) I Andrea Giordano (2) I Xanthi Michail (3) I Nicolas Christodoulou (4) (1) Unit of Occupational Rehabilitation and Ergonomics - Salvatore Maugeri Foundation, Clinica del Lavoro e della Riabilitazione, IRCCS, Veruno (NO), Italy; Past- President of the UEMS PRM Board. (2) Unit of Bioengineering - Salvatore Maugeri Foundation, Clinica del Lavoro e della Riabilitazione, IRCCS, Veruno (NO), Italy. (3) Professor of Rehabilitation Medicine in Physiotherapy Department, Technological University, Athens, Greece; Past-President of the UEMS PRM Board & Incoming President of the European Society of PRM. (4) School of Sciences, European University, Cyprus; President of the UEMS PRM Section & Past President of the Mediterranean Forum of PRM Corresponding Author: Franco Franchignoni, MD, Fondazione Salvatore Maugeri, Clinica del Lavoro e della Riabilitazione, IRCCS, Via Revislate 13, I-28010 Veruno (NO), Italy. Tel + 39. 0322-884.624. Mobile +39. 3395608857. Fax + 39. 0322-830.294. E-mail: franco.franchignoni@fsm.it.