Correlation refers to a regular relationship or association between two dimensions or variables, often expressed in statistical terms. Correlations can be positive or negative. A positive correlation between two variables exists where a high rank or value on one variable is regularly associated with a high rank or value on the other. For example, there is often a positive correlation between years of education and income level. A negative correlation exists where a high rank or value on one variable is regularly associated with a low rank or value on the other. For instance, there might be a negative correlation between the price of a product and the quantity demanded. It is important to note that correlation does not necessarily imply causation; a correlation between two variables may be due to a third, unmeasured variable, or it may simply be a coincidental relationship.