问题如下：
Which of Maxwell’s three concerns can be best described as being attributable to non-stationary distributions?
选项：
A. Concern 1
B. Concern 2
C. Concern 3
解释：
C is correct. A substantial difference in the mean and standard deviation of CSI over the past 20 years relative to those of the past 30 years suggests a change in (i.e., non-stationary) distribution. Even when the data fits a statistical distribution or when historical data distributions are available, shifts in the market structure may lead to shifts in the distributions as well, as evidenced by the shift between the 20- and 30-year distributions of CSI. In some cases, such shifts can change the form of the distribution and in other cases, they can change the parameters of the distribution.
A is incorrect because Concern 1 is a description of real data not fitting distributions. The problem with the real world is that the data seldom fits the stringent requirements of statistical distributions. Using probability distributions that bear little resemblance to the true distribution underlying an input variable will yield misleading results.
B is incorrect because Concern 2 is an issue of correlation that is not incorporated into the simulation. Correlation across input variables can be modeled into simulations. This works only if the correlations remain stable and predictable, however. To the extent that correlations between input variables change over time, as is expressed in Concern 2, it becomes far more difficult to model them.
