16. When the answer (d) σ 2 is unknown, σ is estimated by S, and σ does not appear in the confidence interval. In addition, the confidence interval is 0.95 and it is a bilateral confidence interval, and the quantile is (1-0.95)/2=0.0025.

17. Answer (c). A and b exist, so P(Y=aX+b)= 1, that is to say, y and x are linearly related, so ρ(XY)= 1 (positive correlation) or-1 (negative correlation).

18. Answer (b). The unbiased estimate of sample variance is σ 2.

19. If the answer is (b). If A and B are incompatible with each other, then P(AB)=0, so P(A|B)=P(AB)/P(B)=0 (known as P (B) >; 0)

20. Answer (c). X and y are independent of each other, then p (x =-1| y = 0) = p (x =-1| y = 2) = >; P(X=- 1，Y=0)/P(Y=0)=P(X=- 1，Y = 2)/P(Y = 2)= = & gt； s= 1/ 10

Similarly, from p (x =-1| y =1) = p (x =-1| y = 2) = >; t=2/ 15

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