Where two things are related and one is less likely to happen if the other happens, or, for two numerical variables, when one tends to decrease as the other increases. In the case of numbers, a perfect negative correlation occurs if one variable (say x) is linked to the other (say y) by an exact linear formula such as y = 10 – 2x. However, the effects can also be non-linear, such as y = 10 – 2x^{3} and of course in statistics there is usually a substantial dose of noise! When performing linear regression the Pearson correlation coefficient will be negative.

Used on pages 37, 38, 41