NO.PZ2022071202000068
问题如下：
Question
选项：
A.On average 99% of stock returns will fall within ± 30% from the mean.
B.Bonds have a higher probability of a negative return than do stocks.
C.The probability of a bond return ≤ 3% is determined using a Z-score of 0.25.
解释：
SolutionB is correct. A negative return is any return that is less than zero. 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%. 0.40 of a standard deviation is less than 0.67 of a standard deviation. Negative returns therefore 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.
A is incorrect because on average 95% of returns will fall in the interval μ ± 2σ (which is 30%).
C is incorrect because the Z-score for a 3% bond return is calculated as Z = (X ? μ)/σ = (3 ? 2)/5 = 0.20.
请老师详细解答，对概率的理解还是不到位
