TobyBartels comments on 2014 Less Wrong Census/Survey - Less Wrong

88 Post author: Yvain 26 October 2014 06:05PM

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Comment author: TobyBartels 27 October 2014 03:22:25AM 3 points [-]

Colloquial language doesn't make this distinction, but by technical convention, they are different.

Specifically, ‘odds’ refers to expressions like ‘5 to 3 against’; numerically, that's the fraction 5/3, or rather (because of the ‘against’) its reciprocal, 3/5. Thus odds run from 0 (impossible) to infinity (certain), with odds of 1 being perfectly balanced between Yes and No. In contrast, probabilities run only from 0 to 1. An event with odds of 5 to 3 against, or equivalently odds of 3/5, has a probability of 3/(3+5) = 3/8. So the numbers are different. The conversion formulas are O = P/(1 − P) and P = O/(1 + O).

Then there are log-odds; this is log₂ O bits. (You can also use other bases than 2 and correspondingly other units than bits.) Now 0 indicates perfect balance between Yes and No; a positive number means more likely Yes than No, and a negative number means less likely Yes than No. Log-odds run from negative infinity (impossible) to infinity (certain).

Comment author: Elund 27 October 2014 06:09:07AM *  1 point [-]

Specifically, ‘odds’ refers to expressions like ‘5 to 3 against’

Oh right, I forgot about that definition. The main probability conversions that I was aware of involved converting between fractions and percentages, sometimes expressed instead as probabilities between 0 and 1. Theoretically, it makes sense that odds can also be converted to or from probabilities, now that I think about it. Thanks for your explanation.