The possible values of discrete random variables can be listed. The sum of the probabilities of these possible values is one. The binomial distribution , the random variable that comes from counting heads while flipping coins, is the most useful discrete random variable and is a model for random sampling from a population with two types of individuals. The distribution of a continuous random variable is described by its probability density function .
Probabilities correspond to areas under the curve.
The normal distribution is the most important example of a continuous random variable. This is the distribution of the mean from moderate to large random samples, and also the distribution of random variables in many other examples .