The production (or output or value added) approach. which sums the outputs of every class of enterprise to arrive at the total
The income approach: works on the principle that all of the product must be bought by somebody, therefore the value of the total product must be equal to people's total expenditures in buying things
the expenditure approach: works on the principle that the incomes of the productive factors ("producers," colloquially) must be equal to the value of their product, and determines GDP by finding the sum of all producers' incomes