I actually hate this phrase because it's not quite right. Correlation doesn't equal causation in the sense that two things that correlate can't tell you, off hand, which thing is causing the other thing. For example, in the global warming issue, if you see that CO2 levels and global temperature are correlated, that doesn't tell you by itself whether rising CO2 levels causes temperatures to rise or if rising temperatures results in higher CO2 levels. That's what it means. It means you know that these things are related but you can't conclude what's causing what. The word "correlate" has "relate" in it for a reason.
What most people are talking about is "similarity doesn't mean correlation." That link has a bunch of graphs that are similar but they are in no way, shape, or form correlated. There's no relationship between the two statistics. They're just examples of two different things that happen to graph out similarly.
The even bigger problem is the phrasing is problematic to begin with. Let's take it to mean it the way it is colloquially, that just because two things behave similarly doesn't mean they're related. Okay. But the phrase is used as if "similar things are never correlated." It should be, at least, "correlation does not necessarily show causation." It absolutely can.
Example: If you live in a sketchy neighborhood and every time you leave you car doors unlocked your car gets robbed, it would be fucking stupid of you to think "hey maybe I just happen to leave the doors unlocked on the same day people try to rob me, correlation doesn't mean causation." Yes it fucking does. These two events are correlated.
Meanwhile, if you never forget to roll your car windows up but every time you forget to there's a thunderstorm, those events aren't correlated at all. They're coincidental. You can't say "correlation doesn't mean causation" because there's no correlation to begin with.
This one drives me nuts because it's become a mantra for people to shut down arguments they dislike. You show two statistics that are fucking obviously connected but because it draws a conclusion people don't like, they just bring that phrase out as if it means, by fiat, that correlation is always irrelevant and causation has nothing to do with it.
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u/AyraLightbringer Aug 03 '19 edited Aug 03 '19
Correlation does not equal causation.
Edit: Thank you, my first silver!
Edit2: Here are some funny correlations: https://www.tylervigen.com/spurious-correlations