Patient-reported outcome measurements (PROMs) are often questionnaires which provide and rate the patient's point of view in the measurement of subjective clinical phenomena such as pain or quality of life. In the second of two articles we describe field-testing, the psychometric theories Classical Test Theory and Item Response Theory, psychometric validation and the measurement properties of a PROM. The latter is based on the COSMIN (consensus-based standards for the selection of health measurement instruments) guidelines, which are developed to assist in development and evaluation of PROMs.