Correlation Does Not Imply Causality

Simply because you see X followed by Y many times, or always see X in the presence of Y it does not mean that X causes Y.

My favorite class of example runs along the following lines. There is a strong correlation between aspirin use and tendency to headaches. Nobody is tempted to conclude that aspirin causes headaches.

Brown-eyed people tend to be better at using chopsticks.

Fat people tend to eat at salad bars, therefore salad bars make people fat.

Examples of things that are correlated:

• Test scores of students graduating from universities and the heights of trees on the universities' campuses.
• Ice cream consumption and shark attacks
• Number of children in Danish families and the number of storks observed on their houses.
• IQ scores and foot size
• Skirt lengths and stock prices
• The number of cavities in elementary school children and their vocabulary size.
• LanguageAuthorBeardPattern

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From the film Repo Man (thanks to http://users.erols.com/bobcan/repo/script.html for the transcript!)

Miller

A lot of people don't realize what's really going on. They view life as a bunch of unconnected incidences and things. They don't realize that there's this like lattice of coincidence that lays on top of everything. I'll give you an example, show you what I mean. Suppose you are thinking about a plate of shrimp. Suddenly somebody will say like plate or shrimp or plate of shrimp. Out of the blue, no explanation. No point in looking for one either. It's all part of a cosmic unconsciousness.

Otto

You eat a lot of acid Miller, back in the hippie days?

-- BillBarnett

EditHint: merge with CorrelationImpliesCausation?