An association between data or variables that change or occur together. For example, a positive correlation exists between housing costs and distance from the beach; generally, the closer a home is to the beach, the more it costs. Correlation does not imply causation. For example, there is a statistical correlation between ice cream sales and crime rates, but neither causes the other. The correlation coefficient is an index number between -1 and 1 indicating the strength of the association between two variables.