A quantitative variable whose set of possible values is countable Typical examples of discrete variables are variables whose possible values are a subset of the integers, such as Social Security numbers, the number of people in a family, ages rounded to the nearest year, etc Discrete variables are "chunky " C f continuous variable A discrete random variable is one whose set of possible values is countable A random variable is discrete if and only if its cumulative probability distribution function is a stair-step function; i e , if it is piecewise constant and only increases by jumps