The other day I was musing about a reasonable approach to playing games like the big lotteries. They don't cost a lot and losing $40 is not a life changing event for me, but clearly winning a few hundred million dollars is life changing.
My first thought turned to, well if you just play when the expected value is greater than the cost of the ticket that is "rational". But when I started thinking about it, and even doing some calculations for when that EV condition exists (for things like Mega Millions the jackpot has to be greater then about 550 million) it struck me that the naive EV calculation must be missing something. The odds of actually winning the jackpot are really, really low (as opposed to just really low to rather low for the other prizes). And the payoffs that go into the EV calculation are hugely skewed by the top prices.
I suspect this must be a situation that generalized to other settings and am wondering if anyone knows of better approaches than merely the naive EV calculation. And to be sure I'm using the term as everyone expects, EV just equals the probability weighted payoffs minus the cost of the ticket.
(I'm going to nix the cost of the ticket as it's just a constant)
Depends. Do you want to sum the probability weighted payoffs? EV is fine for that. The probability weighting deals with the striking "really, really low" odds (unless you want to further reweight the probabilities themselves by running them through a subjective probability function), and the payoffs are just the payoffs (unless you want to further reweight the payoffs themselves by running them through a subjective utility function). Either or both of these changes may be appropriate to deal with your own subjective views of objective reality, but that's what they are - personal transformations. However, enough people subscribe to such transformations that EU (expected utility, or see cumulative prospect theory) makes sense more widely than just for you. We indeed perceive probabilities differently from their objective meanings and we indeed value payoffs differently from their mere dollar value.
Now, if you just want a number that best represents the payoff structure, we have candidate central tendencies - mean is a good one (that's just EV). But since the payoff distribution is highly skewed, maybe you'd prefer the median. Or the mode. It's a classic problem, but it's finding what represents the objective distribution rather than what summarizes your possible subjective returns.