英文文献：Estimating Time Variation of Market Power: Case of U.S. Soybean Exports-估计市场力量的时间变化:以美国大豆出口为例
英文文献作者：Nakajima, Toru
英文文献摘要：
As the development of methodology in new empirical industrial organization (NEIO), there have been increasing number of studies that estimate market power of sellers and/or buyers in various kinds of agricultural supply chains. Many of them, however, do not capture time variation of market power during the sample periods, even though they use long time series data. If the degree of market power actually changes along the time, such an estimation that uses only whole sample of the data may provide a misleading conclusion. The main objective of this study is to establish a way of estimating time variation of market power and making a time series index of it. Such an index enables us to make a comparison with indices of market structure, such as market share, which has been one of the main research topics of traditional industrial organization studies. Empirical analysis is conducted using the case of the U.S. soybean exports. The methodology employed in this study to estimate market power of the U.S. exporters is residual demand model, which enables us to derive the degree of market power and has widely been used in the context of international markets. To capture time variation of the market power, rolling window regression method is applied to the model, which is a methodology that repeats regressions using subsamples of total data by shifting the start and end points with a fixed window. Using the parameter of residual demand elasticity in each rolling estimation, a time series index of market power is calculated. The estimation results of rolling regressions of residual demand model using GMM-nonIV and the window size of 30 show that the U.S. had market power over importers to some extent until 1995, but had less and little power from the late 1990s to 2010. It also showed that the U.S. had almost no market power over China from 1996 to 2010. On the other hand, especially from the late 1990s, the U.S. had increasing market power over Mexico and Japan, although the extent was larger to Japan than to Mexico. Using the estimated index of market power and published index of market shares, then the relation between market structure and performance was analyzed. The analysis indicates that the changes in market power of the U.S. soybean exporters to importers average correspond to the decrease of the U.S. market share in the world soybean exports and to the increase of importers concentration due to the increase of China’s imports. It is also pointed out that indices of the U.S. market power over Mexico and Japan who depend soybean imports heavily on the U.S. may correspond to the changes in the market structure of the U.S. grain exporting industry.

随着新型实证产业组织方法的发展，对各类农业供应链中卖方和/或买方市场力量的评估研究也越来越多。然而，尽管它们使用了很长的时间序列数据，但其中许多并没有捕捉到在抽样期间市场力量的时间变化。如果市场力量的程度实际上随着时间的推移而变化，这种仅使用整个数据样本的估计可能会得出一个误导性的结论。本研究的主要目的是建立一种估计市场权力的时间变化的方法，并编制市场权力的时间序列指数。这样的指标可以使我们与市场结构的指标进行比较，如市场份额，这一直是传统产业组织研究的主要研究课题之一。以美国大豆出口为例进行了实证分析。本研究中估计美国出口商市场力的方法是残余需求模型，它使我们能够得出市场力的程度，并在国际市场的背景下被广泛使用。该模型采用了滚动窗口回归方法，利用总数据的子样本，在固定窗口下移动起始点和结束点，进行重复回归。利用滚动估计中的剩余需求弹性参数，计算了市场力的时间序列指数。利用GMM-nonIV和窗口大小为30的剩余需求模型滚动回归的估计结果表明，美国在1995年之前对进口商具有一定的市场影响力，但在90年代后期到2010年，影响力越来越小。它还表明，从1996年到2010年，美国对中国几乎没有市场力量。另一方面，特别是从20世纪90年代末开始，美国对墨西哥和日本的市场影响力不断增强，尽管日本的市场影响力大于墨西哥。利用估计的市场力量指数和已公布的市场份额指数，分析了市场结构与企业绩效的关系。分析表明，美国大豆出口国市场力向进口商市场力的平均变化对应着美国在世界大豆出口市场份额的下降，以及由于中国进口的增加而导致进口商集中度的增加。文章还指出，美国对大豆进口严重依赖于美国的墨西哥和日本的市场力量指数可能与美国粮食出口行业的市场结构变化相对应。