An Implementation for the Gravitational Models of Trade
A system for creating the bilateral trade flow between a country pair equal to zero. You provide the data, tell get_zerotradeflow() which variables are of interest and it expands the base by creating the bilateral zero trade flow. The bases on the flow of trade between countries only report positive trade (greater than zero), however, for some analyzes of gravitacional models, data on zero flow is also necessary. Some examples for Gravity Model: Figueiredo and Loures (2016) <doi:10.5935/0034-7140.20160015> and Yotov, Piermartini, Monteiro and Larch <https://vi.unctad.org/tpa/web/docs/vol2/book.pdf>.
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