The values may not be the same as market values, because there are many incentives to alter them to minimize tax liabilities, and the incentives may be very different for foreign-owned firms from any that domestically owned firms face. Any manipulation of sales values would affect value added even more, and there are incentives to manipulate the profit portion of value added in addition to those affecting sales values. Furthermore, since value added includes profits, it may fluctuate far more over time than any physical measure of production.