What's a Spurious Correlation?
Every sociology major learns about the concept of spurious correlation, but they don’t always fully understand it. This concept matters because when it occurs, two things look like they cause each other, but in reality they don’t.
Here’s how spurious correlation works. Suppose we have two things that are correlated. This means that when we see levels of one of them change, we usually also see levels of the other change. Because we’re academics, and not always very creative, we’ll call these things “A” and “B” (sounds like a Dr. Seuss book).
If we see “A” correlate with “B”
We might be tempted to assume that “A” causes “B” or that “B” causes “A.”
There may be causation, but there may not be. Before we go much further, we have to look for a third variable, boldly named “C”, that creates a spurious correlation between “A” and “B”. This happens when “C” causes both “A” and “B” and thus produces the observed correlation between them.
We might draw it as follows:
Okay, this is where presentations of spurious correlation usually stop. Some students get it, some don’t. I’m teaching research methods this semester, and I wanted to make this concept more understandable, so I asked the students to come up with examples of spurious correlation based on things that they observed in their everyday life. After reading these examples, you should have a better understanding of how spurious correlation works.
- One student had gone out partying the weekend before, and while sitting in the bar watching his friends during the evening, he noticed that people who had the most fun dancing were also those who were most likely to throw up by the end off the evening. It’s not that dancing made them sick (“A” causes “B”), or that being sick make them have fun dancing (yuck—“B” causes “A”), rather there is a third variable, alcohol consumption (“C”) that leads to both fun dancing and sickness.
- A student works as a nurse at a local hospital. He noticed that the patients who received radiation therapy were also those most likely to die (A and B). Why? Cancer (C) leads to both radiation and death.
- A student has noticed that the mornings when she has the toughest time getting out of bed are also the mornings with the most car accidents on the roads around campus. It’s not that her staying in bed causes car accidents (A-> B), or that she stays in bed out of fear of car accidents (B->A); rather bad weather causes both car accidents and her wanting to stay in bed.
- A student lives in the dorms next to some guys who are varsity athletes. The guys were explaining to her that women are attracted to them (A) because they are athletes (B). She, however, pointed out that it could be because of their muscular bodies (C). Maybe being strong increases their athleticism and it also attracts women.
- A student is from the Caribbean island of St. Thomas. He has noticed that the more people there are on the island, the more crime there is. It’s not that simply having a greater population leads to crime, or that crime attracts more people. Instead, the population swells when lots of tourists arrive, and the tourists commit crimes.
- A student explained that when she burps (A), she also gets the chills (B). She doesn’t think that she burps in response to the chills, or that she gets chilled because she burps. Rather, she just thinks that something weird is happening in her body (C) that causes both.
- Finally, a student and his friends formed a rock band some years ago, and they are trying to make it big. It doesn’t look promising though. As he explains it, the more they practice, the better they sound. Unfortunately, he thinks the relationship between practice time (A) and quality of sound (B) is spurious. The band practices in a friend’s unheated garage and during winter its pretty cold, so they don’t like to practice much. When they do practice, their fingers are so cold, that they have trouble playing their instruments. As such, the cold temperature (C) both decreases practice time (A) and performance quality (B), leading to correlation between them. The student doubts that they are actually getting better, and he may need to plan a career as something other than a rock star.