Patient relevant outcomes, such as cognitive functioning and functional status, measured using questionnaires, have become important endpoints in medical studies. Traditionally, responses to individual items are simply summed to obtain a score for each patient. Recently, there has been interest in another paradigm, item response theory (IRT), proposed as an alternative to summed scores. The benefits of the use of IRT are greatest, when it is used in conjunction with a calibrated item bank. This is a collection of items, which have been presented to large groups of patients, whose responses are used to estimate the measurement properties of the individual items. This article examines the methodology surrounding the use of IRT to construct and calibrate an item bank and uses the AMC Linear Disability Score project, which aims to develop an item bank to measure functional status as expressed by the ability to perform activities of daily life, as an illustration.