问题
单项选择题
Given P(X=2) =0.3, P(X=3) =0.4, P(X=4) =0.3. What is the variance of X（）
A.0.3.
B.0.6.
C. 3.0.
答案
参考答案：B
解析：
The variance is the sum of the squared deviations from the expected value weighted by the probability of each outcome.
The expected value is E(X) =0.3×2+0.4×3+0.3×4=3.
The variance is 0.3×(2-3)2+0.4×(3-3)2 +0.3×(4-3)2=0.6.