# Irrational Weighting of Probabilities: A Data Point

by on April 19, 2005

You take your kid to the specialist to talk about persistent symptom x and he says, “Well it’s probably harmless thing A, or maybe harmless thing B. And there’s a very small chance it’s the horrible and ultimately fatal genetic disorder C.”

If I were rational, this conversation would not have upset me as much as it did.

_Update:_ Given all the parents in the world who really do have serious child-health problems to deal with, and worse besides, I now feel ashamed for even bringing this extremely remote possibility of bad luck up here. It wasn’t even meant to be the point of the post, just a springboard for an observation. Don’t be surprised if the post disappears altogether soon, having died of embarassment.

1

Sebastian holsclaw 04.19.05 at 6:09 pm

Ah, but you are human which is to say irrational.

2

ogged 04.19.05 at 6:17 pm

Of course it’s upsetting. Here’s to A or B.

3

Dave 04.19.05 at 6:18 pm

Yeah – what ogged said.

Plus, if you assign a sufficiently negative utility to C, it doesn’t matter how low the probability is. The expectation still sucks.

4

Tom T. 04.19.05 at 6:49 pm

I’m sorry to hear this. Best wishes to you and your child, and here’s hoping that it’s harmless.

5

Kieran Healy 04.19.05 at 7:32 pm

Thanks all. But now I feel a bit embarrassed, given that nothing at all is actually wrong at this point, so far as we know, and the overwhelming likelihood is that there’s nothing to worry about in the future, either. So I should just count my blessings and be quiet.

6

vivian 04.19.05 at 8:07 pm

We love you, whether or not you’re embarassed, and hope for the best.

It’s not the probability, it’s the expected value that has you worried. There is some disutility associated with uncertainty between harmless options A and B, but a massively nonlinear weighting term (negative) multiplies the vanishingly small C probability. It would be really disturbing if you weren’t worried. Though of course people who know you doubtless wish that you could be spared the pain of worrying, that doesn’t mean anyone thinks you’re nuts.

Good luck.

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