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Correlation does not equal causation
Friday, 27 April 2007
We humans are wired to see connections between things. If it rains after the sky gets cloudy, we figure the two are related. If a bad smell arrives at the same time as a person, we decide that he must be the cause of it. If apples fall from trees and hit the ground, we work out that some kind of force must be acting upon them.
Generally, this is a good thing. It's figuring out how all these connections fit together that's a large part of what took us from being cavemen to having all our modern conveniences and knowledge.
But sometimes, these connections can be faulty. And it's this you have to look out for in yourself.
How do we work out that things are connected? Generally it's to do with them occurring together or correlating. For example, if people tend to come into hospital injured immediately following car accidents, you can conclude that the two are related.
But just because things correlate, doesn't necessarily mean they're connected. A good example of this from ancient times were rain ceremonies. In order to grow crops, we need rain. Ancient people knew this, so they tried to influence the coming of the rain. They arranged elaborate ceremonies - from dances to sacrifices.
Sometimes, these ceremonies seemed to work. Soon after they completed it would rain. So the people concluded that the ceremonies had power. Of course, they didn't always work, but people figured this was because they'd made a mistake in the ceremony, not that the two were unrelated.
And of course, certain people had an interest in maintaining the belief that the ceremonies worked. Those who organised and participated in them in particular.
Now, we know that no kind of ceremony is going to affect the rain. We've moved beyond that, but it took a long time. Just because it rains after we hold some kind of dance, we've figured out that the two are simply correlated, and the ceremony isn't the cause of the rain.
But all sorts of superstitions based on correlation survive. You can hear it in many everyday saying such as:
- "Bad things always happen in threes".
- "Only think positive thoughts and positive things will happen".
- "Write down your goals to make them come true".
- And so on.
All of these expressions sound great. Sometimes, they may even come true. But clearly one isn't a cause of the other. They are simply correlated.
Even seemingly educated people can fall into this trap. In my own field of information technology, all sorts of project management techniques have been tried. Sometimes, a particular technique seems to work, simply because the project succeeds. But there are all sorts of factors that contribute to the success or failure of projects. Just because a particular technique was used may be irrelevant. The manager may have just got lucky.
This is how superstitions are formed. One thing appears to come before another in a few instances, so it becomes assumed that the first thing causes the second one.
But there's no reason to assume this is the case. They may simply have got lucky. And the cleverer the connection theory sounds, the more likely it is to be believed.
The trick is to look at a large number of cases to see if one seems to cause the other. And be ruthless about focussing on cases where there seems to be no causal link.
Being able to tell the difference between things that are causes and things that are simply correlated is one of the signs of a sharp thinker. And it's the type of thing you can never be good enough at. Practice improving your own skills in this area.
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