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== References ==

== 参考文献 References ==

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== Further reading ==

== 扩展阅读 Further reading ==

*{{cite book |last=Wooldridge |first=Jeffrey M. |chapter=Policy Analysis with Pooled Cross Sections |pages=438–443 |title=Introductory Econometrics: A Modern Approach |year=2013 |publisher=Thomson South-Western |location=Mason, OH |isbn=978-1-111-53104-1 }}

*{{cite book |last=Wooldridge |first=Jeffrey M. |chapter=Policy Analysis with Pooled Cross Sections |pages=438–443 |title=Introductory Econometrics: A Modern Approach |year=2013 |publisher=Thomson South-Western |location=Mason, OH |isbn=978-1-111-53104-1 }}