skywildworld 最近又翻了下教材，发现事件独立性是这样定义的 P(AB) = P(A)P(B) 但是随机变量的独立性定义是 P(X,Y)=P(X)P(Y),注意这里是二维随机变量分布 但是为什么不能仍然定义为P(XY) = P(X)P(Y)，两者有什么区别吗？
SPSS17 (X,Y) is a pair of 2 dimensional random variable or u can see it as a random vector, but (XY) is an one-dimensional random variable, it can be seen as a scalar, so the difference is SCALAR or VECOTOR.
