Correlation is one of the most common ways of evaluating the relationship between two variables. If one variable increases at the same time another one increases, the two variables are positively correlated. For example, gregariousness and number of friends are well correlated. The more gregarious a person is the more friends they are likely to have. Less gregarious people are likely to have fewer friends. If one variable increases while another decreases, the two variables are negatively correlated. Age and impulsivity are negatively correlated. The older someone gets, the less likely they are to engage in impulsive behavior. Likewise younger people are more likely to engage in impulsive behavior. If there is no relationship between variables, they have no correlation. Month of birth and mathematical skills have no relationship. We do not anticipate that the month of birth would have any impact on a person’s mathematical ability.