The psychometric properties of a rating scale are, unfortunately, badly assessed in practice. Each property is thoroughly studied. Validity has different facets: content, criterion, construct, face validity. Each facet is accurately defined and the experimental and statistical methods used are explained in detail. Reliability is assessed with use of the intraclass correlation coefficient and the Bland and Altman method. Guidelines are given for choosing, in a specific experimental situation, the appropriate coefficient. Responsiveness is evaluated with use of effect size and standardized response mean. Finally practical recommendations are given.