The Spearman correlation, also called the Spearman rank correlation, measures how strongly two variables move together after the raw values have been converted into ranks. Instead of asking whether the points sit close to a straight line, Spearman asks whether higher values of one variable tend to correspond to higher values of the other in a consistent order. The coefficient is written as ρ, and like Pearson's r it ranges from -1 to +1. A value near +1 means the rankings rise together, a value near -1 means one ranking rises while the other falls, and a value near 0 means there is little monotonic association.
The method is named after Charles Spearman, whose work on ranked association made the test a standard tool in psychology, education, and survey research. A Spearman correlationis particularly useful when the variables are ordinal, when the distributions are clearly non-normal, or when outliers could distort a raw-value linear correlation. Because the method works on ranks rather than raw measurements, it is far less sensitive to extreme values and still captures meaningful ordered patterns.
A Spearman rank correlation is also the right choice when the relationship is monotonic but not linear. For example, click-through rate may fall steadily as search rank worsens even though the curve is not straight. In that case Pearson can understate the association, while Spearman still recognizes that the ordering remains consistent. That is why analysts often compare Pearson and Spearman side by side when they need to decide whether they are looking at a straight-line trend or a broader monotonic pattern.