a situation in which variables are associated through their common relationship with one or more other variables but do not have a causal relationship with one another. For example, assume that data show that the total amount of damage in a fire increases as the number of firefighters at the scene increases. One cannot infer from this that calling fewer firefighters would decrease damage: There is a third variable—the initial size of the fire—that influences both the amount of damage and the number of firefighters present. See third-variable problem.