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AlexMennen comments on Median utility rather than mean? - Less Wrong Discussion
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Suppose it comes up with 2 possible policies, one of which involves a 49% chance of death and no chance of injury, and another which involves a 49% chance of light injury, and no chance of heavy injury or death. The median maximizer sees no reason to prefer the second policy if they have the same effects the other 51% of the time.
Er, yes, constructing single choice examples when the median behaves oddly/wrongly is trivial. My whole point is about what happens to median when you aggregate decisions.
You were claiming that in a situation where a median-maximizing agent has a large number of trivially inconvenient action that prevent small risks of death, heavy injury, or light injury, then it would accept a 49% chance of light injury, but you seemed to imply that it would not accept a 49% chance of death. I was trying to point out that this appears to be incorrect.
I'm not entirely sure what your objection is; we seem to be talking at cross purposes.
Let's try it simpler. If we assume that the cost of buckling seat belts is incommensurable (in practice) with light injury (and heavy injury, and death), then the median maximising agent will accept a 49.99..% chance of (light injury or heavy injury or death), over their lifetime. Since light injury is much more likely than death, this in effect forces the probability of death down to a very low amount.
It's just an illustration of the general point that median maximising seems to perform much better in real-world problems than its failure in simple theoretical ones would suggest.
No, it doesn't. That does not address the fact that the agent will not preferentially accept light injury over death. Adopting a policy of immediately committing suicide once you've been injured enough to force you into the bottom half of outcomes does not decrease median utility. The agent has no incentive to prevent further damage once it is in the bottom half of outcomes. As a less extreme example, the value of house insurance to a median maximizer is 0, just because loosing your house is a bad outcome even if you get insurance money for it. This isn't a weird hypothetical that relies on it being an isolated decision; it's a real-life decision that a median maximizer would get wrong.
A more general way of stating how multiple decisions improve median maximalisation: the median maximaliser is indifferent of outcomes not at the median (eg suicide vs light injury). But as the decision tree grows and the number of possible situations does as well, the probability increases that outcomes not at the median in a one shot, will affect the median in the more complex situation.
This argument relies on your utility being a sum of effects from each of the decisions you made, but in reality, your decisions interact in much more complicated ways, so that isn't a realistic model.
Also, if your defense of median maximization consists entirely of an argument that it approximates mean maximization, then what's the point of all this? Why not just use expected utility maximization? I'm expecting you to bring up Pascal's mugging here, but since VNM-rationality does not force you to pay the mugger, you'll have to do better than that.
It doesn't require that in the least. I don't know if, eg, quadratic of higher order effects would improve or worsen the situation.
The consensus at the moment seems to be that if you have unbounded utility, it does force you to pay some muggers. Now, I'm perfectly fine with bounding your utility to avoid muggers, but that's the kind of non-independent decision some people don't like ;-)
The real problem is things like the Cauchy distribution, or any function without an expectation value at all. Saying "VNM works fine as long as we don't face these difficult choices, then it breaks down" is very unsatisfactory. I'm also interested in seeing what happens when "expect to win" and "win in expectation" become quite distinct - a rare event, in practice.
The more concrete argument you made previous does rely on it. If what you're saying now doesn't, then I guess I don't understand it.
I don't follow. Maximizing the expected value of a bounded utility functions does respect independence.
That was an example. There's another one in http://lesswrong.com/lw/1d5/expected_utility_without_the_independence_axiom/ which relies on "not risk loving". That post doesn't mention the median, but it does mention the standard deviation, and we know the mean must be within one SD of the mean (and often much closer).
Choosing to bound an unbounded utility function to avoid muggers does not.
Look, we're arguing past each other here. My logical response here would be to add more options to the system, which would remove the problem you identified (and I don't understand your house insurance example - this is just the seat-belt decision again as a one-shot, and I would address it by looking at all the financial decisions you make in your life - and if that's not enough, all the decisions, including all the "don't do something clearly stupid and pointless" ones).
What I think is clear is:
a) Median maximalisation makes bad decisions in isolated problems.
b) If we combine all the likely decisions that a median maximiser will have to make, the quality of the decisions increase.
If you want to argue against it, either say that a) is bad enough we should reject the approach anyway, even if it decides well in practice, or find examples where a real world median maximaliser will make bad decisions even in the real world (if you would pay Pascal's mugger, then you could use that as an example).
We were modeling the seat-belt decision as something that makes the difference between being dead and being completely fine in the event of an accident (which I suppose is not very realistic, but whatever). I was trying to point to a situation where an event can happen which is bad enough to put in the bottom half of outcomes either way, so that nothing that happens conditional on the event can affect the median outcome, but a decision you can make ahead of time would make the difference between bad and worse.
I do think that a) is bad enough, because a decision procedure that does poorly in isolated problems is wrong, and thus cannot be expected to do well in realistic situations, as I mentioned previously. I guess b) is probably technically true, but it is not enough for the quality of the decisions to increase when the number increases; it should actually increase towards a limit that isn't still awful, and come close to achieving that limit (I'm pretty sure it fails on at least one of those, though which step it fails on might depend on how you make things precise). I've given examples where median maximizers make bad decisions in the real world, but you've dismissed them with vague appeals to "everything will be fine when you consider it in the context of all the other decisions it has to make".
And I've added in the specific other decisions needed to achieve this effect. I agree it's not clear what exactly the median maximalisation converge on in the real world, but the examples you've produced are not sufficient to show it's bad.
My take on this is that counterfactual decision count as well. ie if humans look not only at the decisions they face, but the ones they can imagine facing, then median maximalisation is improved. My justification for this line of thought is - how do you know that one chocolate cake is +10 utility while one coffee is +2 (and two coffees is +3, three is +2, and four is -1)? Not just the ordinal ranking, but the cardinality. I'd argue that you get this by either experiencing circumstances where you choose a 20% chance of a cake over coffee, or imagining yourself in that circumstance. And if imagination and past experiences are valid for the purpose of constructing your utility function, they should be valid for the purpose of median-maximalisation.
That you claim achieve that effect. But as I said, unless the are choices you can make that would protect you from light injury involve less inconvenience per % reduction in risk than the choices you can make that would protect you from death, it doesn't work.
However, I did think of something which seems to sort of achieve what you want: if you have high uncertainty about what the value of your utility function will be, then adding something to it with some probability will have a significant effect on the median value, even if the probability is significantly less than 50%. For instance, a 49% chance of death is very bad because if there's a 49% chance you die, then the median outcome is one in which you're alive but in a worse situation than all but 1/51 of the scenarios in which you die. It may be that this is what you had in mind, and adding future decisions that involve uncertainty was merely a mechanism by which large uncertainty about the outcome was introduced, in which case future-you actually getting to make any choices about them was a red herring. I still don't find this argument convincing either, though, both because it still undervalues protection from risks of losses that are large relative to the rest your uncertainty about the value of the outcome (for instance, note that when valuing reductions in risk of death, there is still a weird discontinuity around 50%), and because it assumes that you can't make decisions that selectively have significant consequences only in very good or very bad outcomes (this is what I was getting at with the house insurance example).
I don't understand what you're saying here. Is it that you can maximize the median value of the mean of the values of your utility function in a bunch of hypothetical scenarios? If so, that sounds kind of like Houshalter's median of means proposal, which approaches mean maximization as the number of samples considered approaches infinity.
The observation I have is that when facing many decisions, median maximialisation tends to move close to mean maximalisation (since the central limit theorem has "convergence in the distribution", the median will converge to the mean in the case of averaging repeated independent processes; but there are many other examples of this). Therefore I'm considering what happens if you add "all the decisions you can imagine making" to the set of actual decisions you expect to make. This feels like it should move the two even closer together.