The input- output table calculates GDP by three interlocking methods. First, it estimates each industry's gross output and subtracts intermediate inputs from other industries to derive each industry's residual value-added, which can be summed in what is sometimes called the "production approach" to estimate GDP. A second approach to estimating GDP, the "income approach" measures the income earned by the different factors of production. The third approach, the "final expenditures approach," shows what is happening across different types of spending such as con- sumption, investment, and exports less imports.

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