Compare two types of specification errors (1) Exclude a relevant variable(the case of underfitting a The coefficients of variables are generally biased as well as inconsistent t The error variance is incorrectly estimated; The standard errors of estimators are biased x The usual hypothesis-testing procedure becomes invalid（1）Exclude a relevant variable(the case of underfitting) ☆The coefficients of variables are generally biased as well as inconsistent； ☆The error variance is incorrectly estimated； ☆ The standard errors of estimators are biased; ☆ The usual hypothesis-testing procedure becomes invalid. Compare two types of specification errors: