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引力方程(gravity equation)是国际贸易的实证研究中最流行的经验型模型，没有之一。
实际执行起引力方程来并不简单，因为贸易数据通常有两个特点：异方差（不同国家的贸易量有不同方差）和自我选择性误差（出口与否是一个国家内生性的决定）。
2014在Applied Economics的一篇文章对比了目前可供使用的所有估计量，为应用引力方程的人提供了一个根据自己手头的数据找到最优估计量的办法。摘要如下：
http://www.tandfonline.com/doi/abs/10.1080/00036846.2014.920481#.VLM0ICvF98E
“Gravity models are widely used to explain patterns of trade. However, two stylized features of trade data, sample selection and heteroscedasticity challenge the estimation of gravity models. We propose a two-step method of moments (TS-MM) estimator that deals with both issues. The Monte-Carlo experiments show that the TS-MM estimates are resistant to various combinations of sample selection and heteroscedasticity. Moreover, the TS-MM estimator performs reasonably well even when the data generating process deviates from the TS-MM assumptions. We revisit the world trade in 1990 to illustrate the usefulness of the proposed model, with emphasis on the identification of the extensive margin of trade.”
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