關稅配額與價格動態反應之結構性變動分析：以大蒜進口為例

大蒜為重要的民生產業，然因國內生產成本高於進口價格，故採取關稅配額方式，以兼顧保護國內生產與支持市場價格。由於關稅配額存在著進口高低關稅之特性，所以國內外價差對於國內價格在進口配額內外之反應時間與程度可能有所不同，本文即以門檻迴歸模型檢定結構轉變與臨界值，並估計配額內外進口的反應落遲時間及對國內價格的衝擊程度。結果顯示結構轉變的臨界值為3,768.12公噸，與進口配額相當接近。在前一期的累計進口量小於或高於臨界值時，國內大蒜價格將受到前5期或前2期國內外價差之影響，且不論前期累計進口量是否超過臨界值，若前期國內價格高於國外價格時，即國內外價差為正時，將推升國內大蒜的價格，也反映我國大蒜市場本質為超額需求的特性。由於存在著結構轉變與前期國內外正負價差的影響不同，本文建議觀察最近月份的累計進口量是否達到臨界值，以作為政府採取緊急或鼓勵進口之決定參考，並建議可適度提高配額，以平穩大蒜價格。  Garlic is a crucial crop in Taiwan and a tariff-rate quota (TRQ) has been implemented to protect domestic production and market price under Taiwan’s WTO agreement, as the cost of the domestic output is higher than the importing price. The TRQ regime imposes lower tariffs within import quotas and higher tariffs outside of them.This study uses threshold regression models to evidence structural changes in the dynamics of garlic price differentials in Taiwan under the TRQ regime. The estimated structural change threshold for the garlic market was 3,768.12 tons, which is close to the regulated tariff quota. This study also reveals that the domestic price of garlic is affected by previous price differentials for up to 5 months if accumulated imports are less than the TRQ and for only 2 months if accumulated imports are greater than the TRQ. The results indicate a tendency for the domestic garlic price to increase when previous price differentials are positive, reflecting excess demand in the garlic market.Due to structural changes and varying impacts of lagged positive and negative price differentials in previous months, this study suggests monitoring the accumulation of imports in recent months to determine whether it reaches the critical value to maintain the stability of garlic prices under the current TRQ regime. In the future, increasing the TRQ level could also help stabilize Taiwan’s domestic garlic market.