Statistics Notes: Validating scales and indexes

An index of quality is a measurement like any other, whether it is assessing a website, as in today's BMJ ,1 a clinical trial used in a meta-analysis,2 or the quality of a life experienced by a patient.3 As with all measurements, we have to decide whether it measures what we want it to measure, and how well. The simplest measurements, such as length and distance, can be validated by an objective criterion. The earliest criteria must have been...