The numbers that are often included in the names of PEGs indicate their average molecular weights (e.g. a PEG with would have an average molecular weight of approximately 400 daltons, and would be labeled PEG 400). Most PEGs include molecules with a distribution of molecular weights (i.e. they are polydisperse). The size distribution can be characterized statistically by its weight average molecular weight ("M"w) and its number average molecular weight ("M"n), the ratio of which is called the polydispersity index ("Đ"M). "M"w and "M"n can be measured by mass spectrometry.