There is much confusion in the understanding and correct usage of correlation and causation. It is easy to make the assumption that when two events or actions are observed to be occurring at the same time and in the same direction that one event or action causes the other. As shown in the 2nd video below, an increase in ice cream consumption in the summer is correlated with increased cases of polio also occurring in the summer. Therefore, eating ice cream causes polio. Or another example: as ice cream sales increase, the rate of drowning deaths increases sharply. Therefore, ice cream consumption causes drowning.
Correlation is an action or occurrence that has a direct link to another action or occurrence; however, this link does not automatically mean that the change in one is the cause of the change in the other. Establishing causality is harder and involves the use of experiments where there are both controlled and independent variables. When the results of these experiments are certain and predictable, causation is established. So, beware, just because two things occur together does not mean that one is the cause of the other.
Video: How Ice Cream Kills! Correlation vs. Causation (5:26)
Video: Correlation vs. Causality: Freakonomics Movie (1:29)
Many normal human beings are not intuitively able to understand the true meaning of correlation and causation. The takeaway message is: When somebody tells you about a relationship between two things, call them A and B, here are the questions you should ask before you agree that there really is a relationship:
- How often do A and B occur together?
- How often does A happen without B also happening?
- How often does B happen without A also happening?
- How often do neither of them happen?