Use it for ordinal data
Survey ranks, ratings, and ordered categories are often better suited to Spearman than Pearson.
Rank correlation
Spearman Correlation
Spearman correlation measures whether two variables move in the same order. It uses ranks instead of raw values, so it is more robust when the relationship is monotonic but not linear.
Use it for monotonic curves
If X rises and Y usually rises, but not in a straight line, Spearman can capture the trend.
Still inspect the plot
No correlation coefficient replaces a scatter plot. Outliers and clusters can still mislead.
Compare methods
Pearson vs Spearman
Choose the correlation method before interpreting r or R².
Interpretation
What R² explains
Use R² when a Pearson linear relationship is appropriate.