Second, the choice of control variables avoids the mentality of random selection. After all, there are many influencing factors of dependent variables, which need to be selected and determined conditionally or according to evidence.
Third, the choice of control variables is not as much as possible. You should know that any economic phenomenon (dependent variable) may be affected by thousands of factors. What we can do is to select some important factors and control them, and the control variables will determine the factors that have an important influence on the dependent variables.
Fourth, the selection of control variables needs to be well-founded, and it can be based on theory or experience first, and the control variables cannot be determined out of nothing.