NO.PZ2017092702000100
问题如下：
An analyst develops the following capital market projections.
Assuming the returns of the asset classes are described by normal distributions, which of the following statements is correct?
选项：
A.Bonds have a higher probability of a negative return than stocks.
B.On average, 99% of stock returns will fall within two standard deviations of the mean.
C.The probability of a bond return less than or equal to 3% is determined using a Z-score of 0.25.
解释：
A is correct.
The chance of a negative return falls in the area to the left of 0% under a standard normal curve. By standardizing the returns and standard deviations of the two assets, the likelihood of either asset experiencing a negative return may be determined: Z-score (standardized value) = (X – μ)/σ Z-score for a bond return of 0% = (0 – 2)/5 = –0.40. Z-score for a stock return of 0% = (0 – 10)/15 = –0.67. For bonds, a 0% return falls 0.40 standard deviations below the mean return of 2%. In contrast, for stocks, a 0% return falls 0.67 standard deviations below the mean return of 10%. A standard deviation of 0.40 is less than a standard deviation of 0.67. Negative returns thus occupy more of the left tail of the bond distribution than the stock distribution. Thus, bonds are more likely than stocks to experience a negative return.
老师这题是不是因为已知了正态分布，所以不用标准化，得出来的概率就可以直接比较大小？
