It is sometimes difficult to determine if a material is non-stoichiometric or if the formula is best represented by large numbers.  The oxides of tungsten illustrate this situation.  Starting from the idealized material tungsten trioxide, one can generate a series of related materials that are slightly deficient in oxygen.  These oxygen-deficient species can be described as , but in fact they are stoichiometric species with large unit cells with the formulas , where n = 20, 24, 25, 40.  Thus, the last species can be described with the stoichiometric formula , whereas the non-stoichiometric description   implies a more random distribution of oxide vacancies.