 PROMs can vary considerably in length and complexity depending on the concept that is being measured. In simple, straightforward cases (e.g., intensity of a certain kind of pain), a single question might suffice. In others, it may be necessary to address several aspects of a more complex functioning with a number of questions each. Either way, the design of the questionnaire should make sure that the instrument reliably measures the concept of interest. The FDA distinguishes the following six measurement properties or “tests” (FDA 2009: 11):