Their scale of measurement is known, they posess scale metric properties, known maxima, minima and reference values, their reliability and validity are known. Metrics are interpreted according to the context in which they were measured. Metrics allow comparisons between applications, as well as between alternative versions of a single developing application, and comparisons with reference values. Conformance to standards and minimal requirements can be tested with metrics. Other metrics predict quality of use factors on the basis of a user interface specification, prototype, or fully functional product, and an underlying user model.
Measurement means a repeatable, objective procedure for generating a measure.
A method is classified to be objective if it is based on objective measurement procedures. However, the data can be of subjective or objective origin. The results of the application of a subjective measurement method are influenced by decisions made by the evaluator in a high degree. Quality factors are features by which a product can be assessed such as efficiency of use, task adequateness, cognitive workload, robustness, learning cost, user acceptance. They are variables which reflect different independent quality aspects of the application.