World Economic Research
Vol.3 No.04(2014), Article ID:14449,7 pages
DOI:10.12677/WER.2014.34006

The Optimization of Export Geographic Concentration of China’s Corn

Yiming Cai

School of Economics and Management, South China Normal University, Guangzhou

Email: fenasl@163.com

Received: Sep. 15th, 2014; revised: Oct. 11th, 2014; accepted: Oct. 17th, 2014

ABSTRACT

Taking stabilizing exports as a goal, in order to optimize export geographic concentration, we should consider volatility risk in each export market. The method of Mean-variance of Markowitz provides a solution for this problem. Since 2005, export volume of China’s corn has showed a falling tendency and obvious volatility. This paper learns from Markowitz model and then constructs an export market mix model available to stabilize export growth rate. According to this model and the data on China’s export between 2005 and 2011, we calculated the effective mixes of China's corn export markets and further obtained the varied geographic concentration corresponding to different expected growth rates of exports. The policy makers will select the optimal geographic concentration in the light of their attitude towards risk.

Keywords:China’s Corn, Relative Variance, Model of Export Market Mix, Export Geographic Concentration, Optimization

Email: fenasl@163.com

1. 引言

2. 中国玉米出口贸易的特征

3. 相对方差与出口市场组合模型

Table 1. The export share in 4 markets of China’s corn: 2005-2011

Table 2. The growth rates of export volume among 4 markets of China’s corn: 2005-2011

Table 3. The correlation coefficients between the export growth rates among 4 markets of China’s corn

3.1. 相对方差及其性质

，其中，为数列的均值，且

3.2. 出口市场组合模型

4. 中国玉米出口市场的有效组合与最优地理集中度

4.1. 有效边界与有效组合

A点所代表的市场组合的预期出口数量增长率为−10.7%，组合相对方差为425.977，二者在所有有效组合中均为最高。在该组合中，朝鲜所占的市场份额最多为99.8%，达到其市场份额的上限。原因在于，其平均出口增长率等于组合市场的预期出口增长率。H点所代表的市场组合的预期出口数量增长率为−80%，组合相对方差为2.636，二者在所有有效组合中均为最低。在该组合中，日本和其他国家的市场份额均达到上限。原因在于，一方面这两个市场的平均出口增长率均接近组合市场的预期出口增长率，同时二者出口增长率的方差均较小(见表5)。另外，在本文选取的8个有效市场组合中，对于日本、朝鲜和其他国家而言，出口份额的上限和下限均出现在不同的市场组合中，这意味着对这3个市场的出口份额在选择不同的市场组合时，面临着较大的调整。

4.2. 最优地理集中度及其含义

Figure 1. The efficient frontier of China’s corn export markets

Table 4. The efficient mix and geography concentration of China’s corn export markets

Table 5. The variance and covariance of the export growth rates among 4 markets of China’s corn

5. 结论

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NOTES

1Hirschman指标的计算公式为，其中为总出口量，为对第个出口市场的出口量。该指标的数值越高表明出口市场越集中。很明显，对出口市场分类标准的不同会影响该指标数值的大小。

22005年以后，中国玉米在韩国、日本和朝鲜以外的出口目的地变化较大，且出口量比较分散，因此本文将其合并为一个“市场”。

3以中国玉米出口为例，在2005年至2011年期间，对韩国的出口增速为−89.4%，而对朝鲜的出口增速则为−10.7%，差异非常明显。

4 G点并不是唯一的反常点。在G点和H点之间，还有很多地理集中度较低而风险却较高的点(表4中没有列出)。