The most commonly used measure of association is Pearson’s product–moment correlation coefficient (Pearson correlation coefficient). The Pearson correlation coefficient* or as it denoted by r** is a *measure of any linear trend between two variables. The value of *r* ranges between −1 and 1.

- When
*r*= zero, it means that there is no linear association between the variables. However, there might be some nonlinear relationship but if*r*= zero then there is no consistent linear component to that relationship.

- When
*r*= 1, it means that there is a perfect positive linear relationship between the variables, and all individuals sampled lie exactly on the line of best fit with a positive slope.

- If 0 <
*r*< 1 then there is a positive linear trend and the sampled individuals are scattered around the line of best fit; the smaller the absolute value of*r*the less well the data can be visualized by a single linear relationship. If*r*is positive then an increase in the value of one variable is associated with an increase in the other variable.

- When
*r*= −1, it means that there is a perfect negative relationship between the variables, and all individuals sampled lie exactly on the line of best fit with a negative slope.

- If −1 <
*r*< 0 then sampled individuals will be scattered around the e variables, and all individuals sampled lie exactly on the; the smaller the absolute value of*r*the less well the data can be visualized by a single linear relationship.

The value of *r*^{2} is called the coefficient of determination. The coefficient of determination is the percentage of variance that could be explained by the two variables.

**Useful resources:**

**References**

Puth, Marie-Therese, Neuhäuser, Markus, & Ruxton, Graeme D. (2014). Effective use of Pearson’s product–moment correlation coefficient. *Animal Behaviour*, 93, 183-189.