 Variables are the basic building blocks of causal models. They will be represented by italicized upper case letters. A variable is a function that can take a variety of values. The values of a variable can represent the occurrence or non-occurrence of an event, a range of incompatible events, a property of an individual or of a population of individuals, or a quantitative value. For instance, we might want to model a situation in which Suzy throws a stone and a window breaks, and have variables S and W such that: