NO.PZ2017092702000086
问题如下：
The total number of parameters that fully characterizes a multivariate normal distribution for the returns on two stocks is:
选项：
A.3.
B.4.
C.5.
解释：
C is correct.
A bivariate normal distribution (two stocks) will have two means, two variances and one correlation. A multivariate normal distribution for the returns on n stocks will have n means, n variances and n(n – 1)/2 distinct correlations.
这道题目问衡量两只同时服从正态分布的的股票，需要几个参数。可以从如下的角度出发分析：一个正态分布有两个参数，均值和方差。所以从两只股票各自都服从正态分布的角度出发，就各自有一个均值和一个方差，一共是四个参数。从两只股票还要同时都服从正态分布的角度出发，还需要有一个correlation的参数，一共就是5个。
为什么还要考虑correlations？
