2. There are two kinds of people, who are those with college degrees and those without college degrees. Assume that they are all white-collar workers. The wage status is described in Table 1. Table 1. The status of two kinds of white-collar workers group sample size college 320 others 685 Questions: mean wage standard error 6.8 4.2 5.6 2.3 (1) Calculate the difference of mean wage between college-degree ones with others. 1 (2) Describe when the difference in (1) could be seen as the causal effect of college degree on wage? (note: randomness) (3) Describe when the difference in (1) could not be seen as the causal effect of college degree on wage?