Correlation or Causality

This is a typical mistake that many people make.  They see correlation and mistake it for causality.


  • Correlation = the degree to which two or more attributes or measurements on the same group of elements show a tendency to vary together.
  • Causality = the principle of or relationship between cause and effect.

I see the difference as correlation shows two things that appear to be related (i.e., I ate a strawberry and had a rash the next day therefore I must be allergic.)  Causality is a direct relationship that is proven where one clearly causes the other (i.e., I went to the allergist and had a bunch of studies done.  I am allergic to strawberries).

There was an article in USA Today called “Many think they have drug allergies” on March 9, 2009.  Apparently many people think they have allergies when they don’t.  In one study discussed in the article, 90% of those people who said they were allergic to penicillin where not when a skin test was done to check.

  • People often mistake side effects with allergic reactions. (e.g., stomach ache)
  • People trust their mothers (i.e., relying on hearsay versus facts).
  • People trust their doctors (when tests may not have been conducted).
  • People grow out of some allergies.

The point of all this is that you need to rely on facts and isolate them to prove causality.  Don’t just look for things that happen at the same time.

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