18. A survey with size of 150 was conducted to investigate whether gender (male versus female) isassociated with annual salary (the dependent variable), after controlling for years of experience, yearsof education, and the position level categorized as "top or middle level manager', "first-linemanager", and "non-manager". You want to conduct a multiple linear regression model. Which of thefollowing is/are correct?
(A) When testing for the overall significance of the model, the degree offreedom for the critical value of F is (6, 143)；
(B) Assuming that we treat "male" as a reference groupfor the gender variable, the estimated coefficient is interpreted as the additional amount of women'sannual salary compared with male's, after years of experiences, years of education, and the positionlevel are held constant；
(C) To test whether years of education is significant factor on annual salaryafter adjusting other variables, we can use t-test with a degree of freedom of 144 to detect whether itsestimated coefficient is different from 0；
(D) Given that the 95% confidence interval of the coefficientfor years of experience is (500, 1000), its estimated coefficient is definitely a positive value；
(E) Thevariable of "position level" has to be converted to a numerical variable by assigning "non-manager"as O, "first-line manager" as 1, and "top or middle level manager" as 2 in order to be included in theregression model.