In statistics, two variables are said to be correlated if the values of one variable are associated with the other variable. Depending on the relationship between two variables, correlation can be of three types– positive correlation, negative correlation, and zero correlation.

Two variables, for example, a and b, are said to be positively correlated if both variables move in the same direction. In other words, a positive correlation exists between two variables, a and b, if:

• Variable a increases as variable b increases
• Variable a decreases as variable b decreases

In a negative correlation, one variable, a, decreases as the other variable, b, increases and vice versa. For example, altitude and temperature are negatively correlated since temperature decreases with the increase in altitude.

Further, when two variables exhibit no relationship, there is said to be zero correlation between them. For example, there is no relation between the number of songs listened to by individuals and their height.

Additionally, correlation can be linear or non-linear. A linear relationship is one where a straight line shows the correlation between two variables. An exponential relationship is an example of a non-linear correlation.