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 biological: the length of a pace, a foot, a thumb. The obvious problem, that the criterion varies from person to person, was eventually solved by establishing a fundamental unit and defining all others in terms of it. Other measurements can then be defined in terms of a fundamental unit. To define a unit of weight we find a handy substance which appears the same everywhere, such as water. The unit of weight is then the weight of a volume of water specified in the basic unit of length, such as 100 …