Using a rank to describe the relationship between data. This data is sorted from the smallest value to the largest, and is given a rank, known as a p-value, to determine if there is a significant relationship between the data.

Data that increases whilst any data related to it also increases is ranked as a positive number; data that shrinks while the other related data grows is ranked as a negative number; and data that has no relationship at all is ranked as 0.

P-values below 0.05 are considered significant and so anything given these values can be said to have a strong relationship.