Correlation is a term that is a measure of the strength of a linear relationship between two quantitative variables (e.g., height, weight). This post will define positive and negative correlations, illustrated with examples and explanations of how to measure correlation. Finally, some pitfalls regarding the use of correlation will be discussed.
Positive correlation is a relationship between two variables in which both variables move in the same direction. This is when one variable increases while the other increases and visa versa. For example, positive correlation may be that the more you exercise, the more calories you will burn. Whilst negative correlation is a relationship where one variable increases as the other decreases, and vice versa.
Where it is possible to predict, with a reasonably high level of accuracy, the values of one variable based on the values of the other, the relationship between the two variables is described as a strong correlation. A weak correlation is one where on average the values of one variable are related to the other, but there are many exceptions.