The two causes of the phenomena give rise to essentially two ways in which creep, stress relaxation, and set tests are used. They can be intended essentially to measure the physical viscoelastic properties, or they can deliberately be used as a measure of aging resistance. The general practice as to which aim is being sought depends on the polymer type and the particular property. Hence creep tests are very often used, particularly with plastics, to obtain the magnitude of the physical effect; stress relaxation of rubbers is commonly used as an aging test, and set of rubbers is most often a quality control test including both physical and chemical effects.