At the core of measurement — What do you want to measure? And how do you want to measure it? A COSMIN perspective

Any measurement starts with determining what to measure (ie, which outcome) and how to measure it (ie, which outcome measurement instrument). This article is the first from a series of the COnsensus-based Standards for the selection of health Measurement INstruments perspective in which we will explain measurements and how to evaluate measurement quality. In this first perspective, we explain the rationale for defining an outcome and operationalizing it into an outcome measurement instrument. An outcome should first be defined based on theory, and next be operationalized into one or more observable items, questions, tasks or parameters. But a measurement instrument is more than its items or machine. Any type of measurement instrument consist of 5 components: (1) equipment, (2) preparatory actions, (3) unprocessed data collection or collection of the biological sample(s), (4) processing and storage, and (5) assignment of the score or determination of the value of the biological sample. Thinking in terms of the components of measurement instruments can enhance the design and understanding of studies on measurement properties.