Glossary term
Correlation
A number between -1 and 1 that says how strongly two quantities move together.
Visual intuition
Correlation is how tightly the dots hug their shared direction
Each dot pairs one day's temperature with that day's ice-cream sales: hot days sit high on both, cool days low — correlation scores how tightly the dots hug that shared drift.
Formula
covariance(x, y) / (sd(x) * sd(y))
Example
If study hours and exam scores have a correlation of +0.8, students who studied more usually scored higher; a correlation of 0 would hours told you nothing about scores (illustrative values).
How It Works
Plot pairs — each dot one day's temperature against that day's ice-cream sales — and correlation scores the pattern: +1 means the dots line up perfectly moving together, -1 perfectly opposite, 0 no straight-line pattern at all. It only measures association: temperature and drownings correlate too, without one causing the other, and it can miss curved relationships completely.
Related Facts
Data Tricks
5 min · easy
Correlation vs causation: why moving together is not proof
Correlation says two things move together. Causation says one changes the other. A hidden third factor can make correlation look causal.
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Does ice cream cause drownings?
Imagine a seasonal chart where ice-cream sales and drownings rise together. Dessert looks suspicious, but the chart alone cannot show cause.
Data Tricks
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Anscombe's quartet has the same statistics and different stories
Four datasets can share almost the same mean, variance, correlation, and regression line, yet look completely different when plotted.