Correlation (r)

A correlation shows how closely two number columns go together. r lies between −1 and +1. At 0 there is no straight-line relationship; positive means more of one goes with more of the other.

Example: Sleep and concentration: r = .19 (115 people). People who sleep more tend to score slightly higher. The relationship is weak.

Caution: A relationship is not a cause. And r only captures straight-line relationships - a scatterplot shows whether that fits.