Usability metric

A number expressing the degree or strength of a usability characteristic, possessing metric properties, obtained by objective counting rules, and with known reliability and validity. Usability metrics have known maxima and minima, their scale of measurement is known, they possess scale metric properties, they are gathered by objectively verifiable rules of counting, and they have demonstrated reliability and validity. They are said to instantiate or operationalise a characteristic of usability. They must be interpreted according to the context in which they were measured.