The evaluation of the outcomes of total knee arthroplasty requires measurement tools that are valid, reliable, and responsive to change. However, the accuracy of any outcome measurement is determined by the validity and reliability of the instrument used. To ensure this accuracy, it is imperative that each instrument used in orthopaedics is free of biases leading to inaccurate estimates of treatment effects. WHERE ARE WE NOW?: Many patient-derived outcome instruments have been developed and tested through the application of the standard assessments that form the basis of classical test theory: validity, reliability, and responsiveness. These assessments determine if the instrument reliably measures what it is intended to measure, and if it captures differences among groups of patients or changes over time. WHERE DO WE NEED TO GO?: Thorough evaluation of the outcome instruments used in orthopaedics is a critical prerequisite for the continued improvement of effective patient care. Additional steps of psychometric testing that are sometimes overlooked include testing for differential item functioning (DIF) and the effects of the mode of administration of the outcome instrument. The use of suitable approaches to test for these potential sources of bias would facilitate the development of more robust outcome assessment in research and clinical practice. HOW DO WE GET THERE?: Testing for DIF, including the effects of mode of administration, may be performed using several analytical approaches. This will allow optimal application of each outcome instrument with respect to patient characteristics, time and mode of the administration, and modification, as necessary.