Friday, March 21, 2008

Fallacy Friday: Sharpshooter Fallacy

On this Fallacy “Good” Friday, we’ll take a look at a logical error that I pointed out in a recent post. It’s formally called ‘non causa pro causa’; I call it the Sharpshooter Fallacy.

The Texas Sharpshooter is a fabled marksman who fires his gun randomly at the side of a barn, then paints a bull’s-eye around the bullet holes – to obviously make it look like he is an amazing shot. The fallacy is applied to the field of logic to describe a false effect after-the-fact. Information that has no relationship is interpreted or manipulated until it appears to have meaning.

Seeing a Face in the Clouds
This fallacy is similar to one of our earlier fallacies, post hoc ergo propter hoc. The fallacy is to assert that because two events occur together, they must be causally related. It's a fallacy because it ignores other factors that may be the cause(s) of the events.

What appears to be statistically significant (not due to chance) is actually expected by the laws of chance. This is probably best shown in the act of reading tea leaves or casting bones, as we’ve read about in old tales. The positioning of the leaves or bones isn’t significant or special; it’s just an artifact of randomness. The fallacy occurs when people try to place unreasoned meaning upon these artifacts.

What a Coinkydink!
When someone jumps to the conclusion that a cluster in some data must be the result of a cause--usually one that it is clustered around--they’ve fallen for the sharpshooter’s trick. When looking at data, there is a danger of jumping to a conclusion that a random cluster is a causal pattern.

What makes this fallacy dangerous is that, to the untrained eye, it can look a lot like what real scientists really do when they look at data and draw conclusions. There are other tests you can run to find the probability of a certain thing, but this is often overlooked by those who *want* there to be meaning where there is none (i.e., the self-made prophecy).

This problem is at the heart of the fine-tuning argument as scientific evidence for a god. There’s no reason to think that the universe is fine-tuned for life – actually, evolution shows that life is fine-tuned for the universe! The ‘fine-tuned’ argument is just like throwing six, six-sided dice. They come up, say ‘3, 6, 3, 2, 1, 1’. The odds of that happening are 1 in 1,679,616 chance!!! The fallacy is (obviously) that you’re counting it up after-the-fact. It’d be like saying “what are the odds that we’d draw the letter A the way we do?” like it’s got some sort of significant meaning. Without further testing, such a conclusion is seldom justified.


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