All of the models previously discussed involved only one random variable and were called univariate models. We now move forward and discuss probability models that involve more than one random variable, called multivariate models. Most experiments involve more than one variable. Thus it is important to know how to describe and use probability models that deal with more than one random variable. In this lesson, we discuss only the discrete case. At the beginning of the next week, we continue the discussion by considering the continuous case.