Slippage in the Conservation Reserve Program or Spurious Correlation? A Rejoinder
回应Wu（2005）关于保护储备计划（CRP）中“滑移”现象的研究，指出其工具变量（土壤侵蚀性）可能受遗漏变量干扰，导致Hausman检验结果不可靠。
The Conservation Reserve Program (CRP) pays farmers about $2 billion per year to retire cropland under ten‐ to fifteen‐year contracts. Recent research by Wu (2000) found that slippage—an unintended stimulus of new plantings—offsets some of CRP's environmental benefits. In a comment on Wu, we argued CRP enrollments were endogenous and confounded by omitted variables. In his reply, Wu (2005) used results from a Hausman test to argue that CRP enrollments are exogenous. In this rejoinder, we explain why the candidate instrument (erodibility) is likely confounded by omitted variables, so Wu's use of the Hausman test is uninformative.