4 ms·

For probabilities, use Fermi numbers, not words

17 points by cpeterso 3y ago

skimdesk 3y ago
Consider a sentence like "We will finish project x next week with 75% probability". I'm not sure it is any more clear than "It is probable that we will finish project x next week". The point estimate is of little value unless we know how it was made. In this case "Jim just came up with a number". Instead of talking of probability, an estimate of the number of days/hours/tasks would seem more grounded and easier to come up with. In this case I'd go for, "We will finish project x after we have completed four days worth of work".
- necovek 3y ago
  That certainly helps in eliminating some hard-to-control-for variables (like how much work time you will really have, as in what things will pop up that need urgent attention), and it also gives you "error bars" by default (4 days means 3-5 days), but lots of projects involve a bunch of unknowns, making them impossible to estimate with any sort of precision before you actually dive into them. As with anything, estimations should be iteratively improved, but usually nobody bothers ;)
- deleted 3y ago
  [deleted]
necovek 3y ago
I disagree fully. Putting a made up number is not brave, just... foolish. A number always includes "precision": if I say 60% chance, I am suggesting that it's between 55% and 65% (or 50%-70%, depending on how you read significant digits). If I say 63%, it signals that I am pretty certain it's in the 62.5%-63.5% range. Sure, even this is sometimes not enough (60% can also be more precise at 59.5%-60.5%), but how many significant digits you use gives you a default error bar too! When we use words like probable and improbable, we have very large error bars. And yes, they are not symmetric, and that's fine too. The article is right that we use this type of language because we don't know actual probabilities, but that's exactly why we don't put numbers on them. There is nothing brave in saying "70-80%" instead of "probable" when that probability is 70%±50% (iow, we might be totally wrong that this is probable; obviously, can't go higher than 1 :).

17 points by cpeterso 3y ago

Consider a sentence like "We will finish project x next week with 75% probability". I'm not sure it is any more clear than "It is probable that we will finish project x next week". The point estimate is of little value unless we know how it was made. In this case "Jim just came up with a number". Instead of talking of probability, an estimate of the number of days/hours/tasks would seem more grounded and easier to come up with. In this case I'd go for, "We will finish project x after we have completed four days worth of work".

necovek 3y ago
That certainly helps in eliminating some hard-to-control-for variables (like how much work time you will really have, as in what things will pop up that need urgent attention), and it also gives you "error bars" by default (4 days means 3-5 days), but lots of projects involve a bunch of unknowns, making them impossible to estimate with any sort of precision before you actually dive into them. As with anything, estimations should be iteratively improved, but usually nobody bothers ;)
deleted 3y ago
[deleted]

necovek 3y ago

I disagree fully. Putting a made up number is not brave, just... foolish. A number always includes "precision": if I say 60% chance, I am suggesting that it's between 55% and 65% (or 50%-70%, depending on how you read significant digits). If I say 63%, it signals that I am pretty certain it's in the 62.5%-63.5% range. Sure, even this is sometimes not enough (60% can also be more precise at 59.5%-60.5%), but how many significant digits you use gives you a default error bar too! When we use words like probable and improbable, we have very large error bars. And yes, they are not symmetric, and that's fine too. The article is right that we use this type of language because we don't know actual probabilities, but that's exactly why we don't put numbers on them. There is nothing brave in saying "70-80%" instead of "probable" when that probability is 70%±50% (iow, we might be totally wrong that this is probable; obviously, can't go higher than 1 :).