What makes a measurement instrument valid and reliable?

High quality instruments are useful tools for clinical and research purposes. To determine whether an instrument has high quality, measurement properties such as reliability and validity need to be assessed, using standardised criteria. This paper discusses these quality domains and measurement properties using the standardised criteria that were recently published by the COSMIN group. Examples are given of studies evaluating the measurement properties of instruments frequently used in trauma. This paper presents a helpful tool for readers who want to evaluate or assess the quality of a measurement instrument on reliability and validity.