Production occurs in stages: some firms produce outputs that are used as inputs by other firms, and these other firms, in turn, produce outputs that are used as inputs by yet other firms. If we added up all the market values of all outputs of all firms, we would obtain a total greatly in excess of the value of the economy"s actual output. Double counting is the error that would arise in estimating the nation"s output by adding all sales of all firms. Intermediate goods - all outputs that are used as inputs by other producers in a further stage of production. Final goods - goods that are not used as inputs by other firms but are produced to be sold for consumption, investment, government, or export during the period of consideration. It is extremely difficult to distinguish final from intermediate goods.