 Statistics related to material flow accounting are usually compiled by national statistical offices, using economic, agricultural and trade statistics measuring the exchange of material between different products available in an economy.Aside from calculating the "net additions to stock" (NAS) as a balancing item, flows within the economy are not considered (advances are currently being made in the field of dynamic stock modelling).MFA covers all solid, gaseous, and liquid materials, mobilized by humans or by their livestock, with the exception of bulk water and air.