A few years ago, a school board discovered that there was a positive correlation between a student’s shoe size and a student’s intelligence. Those with larger shoe sizes had higher IQ scores than those of smaller shoe size. The school board administrators, having little knowledge of statistics, assumed that there was some kind of causal connection between shoe size and intelligence.
However, it was later discovered that the two variables are independent and were merely connected to a third variable, age. Older students, on average, scored higher than younger students on the same IQ test and, of course had larger shoe size. The two variables, therefore showed a positive correlation although they were not causally related.
Similarly, in cities off the coast of New Jersey, it was found that there was a correlation between the number of shark bites and sales of ice cream. This occurred because both ice cream sales and shark attacks increase during the summer, though there is no causal connection between the two.
