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诊断检验部分，给出三个检验：weak instruments(弱工具检验）；Dubin-Wu-Hausman（内生解释变量检验）；Sargan（工具变量的外生性检验，只有工具变量的个数多于解释变量个数时，才可检验）

Should diagnostic tests for the instrumental-variable regression be carried out? These encompass an F test of the first stage regression for weak instruments, a Wu-Hausman test for endogeneity, and a Sargan test of overidentifying restrictions (only if there are more instruments than regressors).