Tijt refers to the export volume from country i to country j in year t. FDIit is the foreign
direct investment (FDI) that country i gains. NUMit is the number of various RTAs that
country i has joined. POPit is the population of country i, which helps determine the effect
of a country’s labor on its exports. STRUit is the ratio of country i’s exported industrial
finished products to its exports. The proportion of finished products in the trade structure
indicates whether a country can launch intra-industry trade to form close trade ties. Intraindustry
trade level has become the driver of regional trade growth and the indicator of
regional integration level