You probably learned somewhere that correlation does not equal causality. That is to say, just because two variables rise and fall together doesn’t mean that one causes the other. One of the examples is that ice cream cone sales rise and fall with rape rates. It isn’t that ice cream cones cause rape; it is that high temperatures increase demand for ice cream and cause many people to leave windows open to get a cool breeze to enter; instead, a rapist takes advantage of the unlocked window.
In that case, there is a common factor driving both variables: high temperatures. Sometimes the correlation is just a coincidence. Statisticians have ways to figure out whether correlations are coincidental or not — but even then, the best that you can say for any particular correlation is that it is unlikely that the correlation is coincidence.
The bigger problem is when you have a correlation that is not a coincidence, but the question becomes, “Which is the cause? Which is the result?” There’s a strong correlation between smoking and lung cancer, but longitudinal studies have pretty well established that smoking comes before the lung cancer. That’s a pretty strong clue that smoking causes lung cancer — not that people who get lung cancer take up smoking because they are so upset about being sick.
Similarly, if you surveyed American homes, you would find a strong correlation between the presence of legally prescribed syringes and at least one diabetic in the home. If you didn’t know that diabetics use syringes to inject insulin, you might wonder: Do syringes cause diabetes? Or does diabetes cause syringes to appear? This problem is known as determining the direction of causality.
Unlike “chicken or the egg,” longitudinal studies give you a realistic chance of figuring out, when you have two correlated variables, which is the cause and which is the symptom. All this discussion is to introduce one of those reminders that correlation is not enough. There’s a new study coming out, just in time to try and influence the Supreme Court in the upcoming challenge to Chicago’s handgun freeze law.