statistical relationship where two variables change together
Pearson coefficient (r) ranges from -1 (perfect negative) through 0 (none) to +1 (perfect positive)
positive correlation: both variables increase together
negative correlation: one increases as the other decreases
does not imply causation: confounders, reverse causation, or coincidence may explain the association
Spearman rank correlation handles nonlinear monotonic relationships
spurious correlations arise from large datasets with many variables
partial correlation controls for the effect of a third variable
foundational tool in statistics, epidemiology, economics, and machine learning