There is an old saying: "Correlation does not mean causation". When I teach, I tend to use the following standard examples to illustrate this point:
- number of storks and birth rate in Denmark;
- number of priests in America and alcoholism;
- in the start of the 20th century it was noted that there was a strong correlation between 'Number of radios' and 'Number of people in Insane Asylums'
- and my favorite: pirates cause global warming.
However, I do not have any references for these examples and whilst amusing, they are obviously false.
Does anyone have any other good examples?
Best Answer
It might be useful to explain that "causes" is an asymmetric relation (X causes Y is different from Y causes X), whereas "is correlated with" is a symmetric relation.
For instance, homeless population and crime rate might be correlated, in that both tend to be high or low in the same locations. It is equally valid to say that homelesss population is correlated with crime rate, or crime rate is correlated with homeless population. To say that crime causes homelessness, or homeless populations cause crime are different statements. And correlation does not imply that either is true. For instance, the underlying cause could be a 3rd variable such as drug abuse, or unemployment.
The mathematics of statistics is not good at identifying underlying causes, which requires some other form of judgement.