M.10—Correlation, causation, Pearson and Spearman

Pearson and Spearman coefficients describe relationships from −1 to +1, but neither alone establishes causation.

Choose the test that matches data type and pattern, inspect confounding variables and state conditions for validity.

Plant abundance may correlate with soil moisture because both respond to altitude rather than because moisture alone causes the pattern.

A strong correlation can be spurious, and a weak correlation can hide a non-linear relationship.