Worthless Evidence

Worthless Evidence

Dennis Bradford

388 Posts



Worthless evidence sometimes negatively affects our judgments.

As any even mediocre epistemologist can confirm, nobody has a clear concept of nondemonstrative evidence.  So nobody understands what its worthless variety even is, much less the damage it can do.

What’s interesting is that that variety is able to affect our judgments negatively. It seems as if we use a small number of heuristic principles that enable us to simplify complexities.

If we lacked such principles, we’d be in worse shape than we actually are when making judgments, particularly judgments about what to do or not to do. The reason is that they often work quite well.

On the other hand, they can lead to important, systematic errors.  It’s hardly surprising that oversimplifying doesn’t always work well.

What’s interesting is that, because the errors that it leads to are systematic, if we are careful and thoughtful it’s possible to avoid them.

For example, worthless evidence doesn’t have to skew our judgments unfavorably – yet it does when it comes to resemblance or representativeness. It’s been shown experimentally that, when we lack worthless evidence in a certain kind of case, our judgments will be correct and that, when we have worthless evidence about the same case, our judgments will be incorrect!

Suppose that there are 100 people in a group.  70 are lawyers and 30 are architects.  What is the probability that one individual selected at random is a lawyer?

Everyone unhesitatingly thinks it .7.  That’s correct.

However, the situation changes when some irrelevant information is added.

Suppose we select an individual at random from the same group.  Suppose someone is told that this person, let’s call him ‘Jim,’ is 30 years old, single, childless, and talented.  Furthermore, Jim is well-liked by his colleagues and it appears that he will be very successful in his field.  What is the probability that Jim is an architect?

People in fact say it is .5!  That’s incorrect.

The correct answer is the same as before:  .7.

In cases like these, irrelevant information negatively affects judgments.


It’s likely because of resemblance to a preexisting stereotype.  In this case, presumably Jim resembles the stereotype of an architect more than the stereotype of a lawyer. Whether or not Jim’s description resembles the stereotype of an architect more closely than it resembles the stereotype of a lawyer is irrelevant.  However, prior probabilities are ignored when such irrelevant information is provided.

It’s like a wrench that falls into the works and causes them to go awry.

When people are asked to judge the probability that Jim belongs to a certain subset or class, their judgments can be negatively affected by worthless information.

Usually, our everyday heuristic principles work well enough.  However, when we are making important decisions, it’s wise to be more critical of our own thinking.

Epistemologists will be unsurprised by this, but it may come as a surprise to many people.

Related post:  Epistemology.

Suggestion for Further Reading:  Amos Tversky and Daniel Kahneman, “Judgment Under Uncertainty:  Heuristics and Biases,” Science, vol. 185, 1974; reprinted in Kahneman’s Thinking, Fast and Slow.

1 thought on “Worthless Evidence

  1. Whyaxye

    “What is the probability that Jim is an architect?

    People in fact say it is .5! That’s incorrect.

    The correct answer is the same as before: .7.”

    I would have thought that the correct answer was not the same as before, because the question has changed. First time, it was the odds of a person being a lawyer. Second time, an architect.

    13/03/2012 at 2:11 pm

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