After adding a control variable, the sign of the independent variable will be reversed. This is because the change of the control variable leads to noise or other information in the main variable factor, which leads to the sign inversion of the independent variable. Adding control variables may lead to the sign inversion of independent variables, because the change of control variables leads to noise or other information in the main variables, which leads to the sign inversion of independent variables. In order to avoid this situation, we should ensure that the information affecting the main variable is realized when adding the control variable, and ensure that all interference factors are eliminated.