http://1038.edu.pinggu.com/forum ... c317/homework-5.pdf, 最后一段有提到,这里面的例子也是最后vce (robust)后不显著了。For the given dataset, robust values of standard errors lead to very small critical values for significance tests, so no one of the coefficients is statistically significant. Thus, one-step regression gave us statistically significant coefficients and the two-step regression – and its tests – confirmed that then instrumental; variables used here are valid.