问题如下:
A fund manager finds that the return of funds in the market are normally distributed having a mean of $160,000 and a standard deviation of $30,000. What is the percent of return that are less than $100,000 ?
选项:
A. 0.05%.
B. 0.13%.
C. 2.28%.
解释:
C is correct.
z = (100000- 160000) /30000 = -2,
F(-2) = 1 - F(2)= 1 - 0.9772 = 0.0228
課代表請問我這個解題思路是否正確? 我利用confidence interval 排除了其他兩項,因為是z算出是-2(2倍),所以按照倍數可以確定在99%(2.58倍)和95%(1.96倍)之間的區間,因為左右對稱,按照99%的剩餘尾部左右兩邊概率是0.5%,而96%是2.5%,所以在選項中可以在這區間的(0.5,2.5)只有一個。