Opponents argue the opposite.
Neither side is right. Schiff shows that neither analysis is correct. Schiff asks not whether a country should form or join a trading bloc but which partner or partners it should select if it does join such a bloc. He shows that the pre-PTA volume of trade is not a useful criterion for selecting a partner. The pre-PTA volume is equal to zero if the partner is an importer of the good sold to the home country and it is indeterminate if the partner is an exporter of that good.