In clinical medicine numerous measurement instruments have been developed to assess the functional outcome of patients in clinical trials. The majority are questionnaires with multiple-choice items. The responses of the patients are used to calculate a sum score. However, there are some disadvantages associated with the use of sum scores. Modern clinimetric methods, based on item response theory (IRT) in combination with a calibrated item bank, can overcome these problems. When using this measurement technique, it is possible to arrange both the item difficulty and the patient's ability on a single, hierarchical linear scale. This allows the user to obtain a sufficiently detailed clinical picture using a small number of items. In addition to being efficient, this method makes it possible to present different sets of items to different groups of patients. Since all of the items are calibrated, the measurements remain comparable. The application of this innovative method of measuring is being studied in the ongoing 'Amsterdam linear disability score' (ALDS) project.