Answer to Question 1
Answer: First obtain a cross-tabulation for the dependent variable and the independent variable. Then, for each value of the control variable, get a separate cross-tabulation table showing the relationship between the dependent variable and the independent variable. Compare the relationships in the different tables. If the relationships are substantially different from one another or from the original relationship (before controlling), then the control variable (the antecedent or intervening variable) makes a difference in the relationship between the dependent variable and the independent variable. If the relationships are basically the same in the different tables (including the original table before controlling), then the control variable is not really an antecedent or intervening variable; it does not affect the relationship between the dependent variable and the independent variable.
Answer to Question 2
Answer: When we think that a variable might affect the relationship between the dependent variable and the independent variable, we want to control for that variable to see whether it has any effects. When we control for this variable, it is a control variable, but we dont know whether it actually has an effect until after the analysis. If it has an affect on the relationship, then it is an antecedent or intervening variable. If not, it was still a control variable.
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