看文献时，时常有疑问。向原文作者讨教应该是“捷径”，可惜呀，大部分论文留的通讯作者和邮箱，根本不会回复。中文邮件经常碰壁，试试用英文向发表在国外期刊的中文作者请教呢，多半也是石沉大海。以下是近期的一封邮件，像投稿一样，无人理会，没有盼头，白白等待。“Dear ***, it is my pleasure to read your paper of "Development of an SBM-ML model for the measurement of green total factor productivity:The case of pearl river delta urban agglomeration". We intend to cite this research.There is only one practical question we need to have a check. When we calculate the GDE_base by SBM and MaxDEA 9.0, which button should we choose, input-oriented, output-oriented or non-oriented? As for ML,we also need to make such a hard choice. In our opinion, ML by Chung et al.(1997) is based on a directional output distance function, in order to keep consistence we should choose output-oriented button for both SBM and ML. Is that appropriate? We are looking forward to your advice.Thank you in advance.”