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Correlation and Causation

Correlation means that two trends go together; causation means that one of those trends causes the other.

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Ice Cream Causes Polio

Ice Cream Causes Polio

Credit: Centers for Disease Control and Prevention
Source: http://en.wikipedia.org/wiki/File:Polio_physical_therapy.jpg
License: CC BY-NC 3.0

Polio once crippled thousands of people, mostly children. Polio epidemics were during summer and fall. People eat more ice cream during summer and fall. So for a while, children were warned not to eat ice cream or they would get polio.

Why It Matters

  • Correlation and causation are not the same thing.
  • Correlation is worth nothing. It just means two trends are going in the same direction.
  • If two trends are correlated, one may be causing the other to happen. This is causation.
  • People waste a lot of time and may do damage trying to stop things that are only linked by correlation.
  • Sometimes the relationship is backwards. People think A causes B, but really B causes A.
  • Sometimes, as in the polio and ice cream example, they are related by a third factor. Polio and ice cream consumption both increase in the summertime. Summer is when the polio virus thrived. Check out the short clip about this example below: http://youtu.be/lbODqslc4Tg

Can You Apply It?

With the link below, learn more about correlation and causation. Then answer the following questions.

  1. Correlation: Marriage (A) and longevity is (B). Describe the correlation (do not assign a cause). Which did people think was the cause of the other? What is more likely to the cause of the other? Or is there a third factor; if so, what is it?
  2. Correlation: Sleeping with the lights on is (A) and shortsightedness (nearsightedness) is (B). Describe the correlation (do not assign a cause). Which did people think was the cause of the other? What is more likely to the cause of the other? Or is there a third factor; if so, what is it?
  3. Correlation: Kids who do well in school is (A) and high self esteem is (B). Describe the correlation (do not assign a cause). Which did people think was the cause of the other? What is more likely to the cause of the other? Or is there a third factor; if so, what is it?
  4. Why does she say that the conclusion that high self-esteem causes kids to do well in school is the worst example of incorrectly identifying cause?

Image Attributions

  1. [1]^ Credit: Centers for Disease Control and Prevention; Source: http://en.wikipedia.org/wiki/File:Polio_physical_therapy.jpg; License: CC BY-NC 3.0

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