大台北地區蔬菜逆需求體系及數量彈性之估計

Demand for Vegetables in the Greater Taipei Area: A Complete System of Quantity Effects on Prices

本研究採用一階微分近似法（first order differential approximation），設定大台北地區蔬菜逆需求體系之實證模型，並在估計時加入齊次性、對稱性、庫諾加總性以及規模加總性四個限制條件使估計出來之結果符合需求理論的要求。本研究將蔬菜分成五大類，分別為根菜、莖菜、葉菜、花果菜以及其他菜類，實證資料為民國八十九年台北市第一、二果菜市場合計的日交易價、量資料，並以迴覆似乎無相關迴歸法來估計逆需求體系。本研究估算出的自身受補償數量彈性均為負數，而規模彈性亦為負數，均符合需求，理論的要求，也符合一般的預期，至於交叉受補償數量彈性則有正有負，顯示不同產品間有數量上替代及數量互補的情形。在未受補償數量彈性方面，自身及交叉未受補償數量彈性均呈負數，而自身未受補償之數量彈性絕對值，以葉菜類最大，根菜類最小，此一結果與一般均認為國人消費習性偏好葉菜類的看法相一致。此外因根菜類數量對其他蔬菜價格之交叉未受補償數量彈性之絕對值均很小，所以即使根菜類較易貯存，若要利用貯藏根菜類，來平抑其他蔬菜類不足而引起的價格上漲，效果將非常有限。

This study applies the first order differential approximation approach to specify an inverse demand system model. To narrow the gap between demand theory and empirical application and to provide greater statistical efficiency to the estimated demand parameters, properties including homogeneity, symmetry, Cournot aggregation and scale aggregation were incorporated into the empirical model.The study divides vegetables into five categories, namely root vegetables, stem vegetables, leafy vegetables, fruit vegetables and others. The daily time series data of volumes and prices were obtained from the statistics of Taipei fruit and vegetable auction market in 2000. The iterative seemly unrelated regression method was used to estimate the parameters.Both the compensated self-quantity elasticity and scale elasticity of each vegetable category were negative, which were consistent not only with the theoretical requirement but also with the general expectation. The signs of compensated cross-quantity elasticity were mixed, which revealed that there existed the complement and substitute relationships between different pairs of vegetables.All the uncompensated self and cross quantity elasticity were negative. Meanwhile, the absolute value of self-quantity elasticity of leafy vegetables was the highest and that of root vegetables was the lowest. This result is consistent with the viewpoint that consumers in Taiwan prefer leafy vegetables to root vegetables. Furthermore, the absolute value of the uncompensated cross-quantity elasticity of each vegetables with respect to root vegetables were very low, indicating that stocks of root vegetables will not be effective in depressing the sharp rise in price of other vegetables during the typhoon season.