Glossary term
Confounder
A hidden third factor that influences both things you are comparing, creating a link that is not causal.
Visual intuition
A hidden third factor can move both quantities at once
Each dot is one week: ice-cream sales paired with drownings. They climb together — not because one causes the other, but because summer pushes both up.
Example
Early studies found coffee drinkers had more lung cancer; the confounder was smoking, which was more common among coffee drinkers and caused the cancer.
How It Works
Ice-cream sales and drownings rise and fall together — not because ice cream drowns anyone, but because summer drives both. Summer is the confounder: a backstage variable moving both quantities at once, so they dance in step without touching. Confounders are why 'people who do X are healthier' headlines mislead — people who do X often differ in a dozen other ways — and why studies control for age, income, and season, or randomize to break the link.
Related Facts
Data Tricks
4 min · easy
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
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.
Paradoxes
4 min · medium
A trend can reverse when groups are combined
A treatment can look better in every subgroup but worse overall.