Molecular-weight determination methods can be classed as absolute, equivalent, or relative. Absolute methods allow the molecular weight to be calculated directly from the measured quantities; a knowledge of the physical and chemical structure of the molecules is not required. Absolute methods include colligative methods (membrane osmometry, ebulliometry, cryoscopy, and vapor-phase osmometry), light scattering, and equilibrium sedimentation. Some methods yield simple, others mixed, averages of the molecular weight. Colligative methods indicate the number of molecules, thus leading to a number-average molecular weight.