jwgh: (Default)
Jacob Haller ([personal profile] jwgh) wrote2006-10-06 01:25 pm
Entry tags:

contest

Pick a number, any number. I'll send the person who picks the number closest to the average of all the entries $5. (Average in this case means the arithmetic mean.)

If more than one person wins I'll pick one of them at random.

The winner will be chosen on Monday, at which point I'll also reveal the numbers that people picked. (For now, for obvious reasons, the results of the poll can only be viewed by me.)

Numbers chosen must be real numbers. Please don't submit entries that make me do complicated calculations, or look things up, etc.

[Poll #838512]
Examples:

If three people enter the contest, and they pick 5, 11, and 23, then the average would be 13.0, and the person who picked 11 would win.

If the only entries were pi, -12, seven billion, and 2 1/2, then the average would be around 175 million and the person who picked pi would win.
Where this idea came from:

I had heard that, in contests of the sort where there is a fishbowl full of jelly beans and people have to guess how many beans are in the bowl, although individual guesses may be off wildly, the average of the guesses is likely to be very close to the number of beans, and indeed will often be closer than any single guess.

So it occurred to me that if you were holding a contest of this kind, it would probably be more practical to calculate the average of the guesses (since you have to go through all the entries anyway) and use that as the assumed number of beans than to actually empty the beans and count them. And then I thought, at that point, why do you need the bowl or the beans to begin with?
Why I am doing this:

I am curious as to what numbers people will pick.

[identity profile] mskala.livejournal.com 2006-10-06 05:42 pm (UTC)(link)
This reminds me of the way prices are set in the stock market. Nobody really knows how much a share is worth; that is to some degree unknowable. But people will try to guess how much the share is worth, and then buy or sell it if the prevailing price is noticeably more or less than their estimate, and it's widely believed that the prices set by that process accurate reflect the value of the business.

But after a while it can turn into a game of buying and selling where nobody really cares about the underlying business anymore, they're just trying to guess the averages that will come out of the marketplace process... and then that's the situation where you dispense with the jar of jelly beans.

You game has an interesting wrinkle to it if we start thinking about players trying to manipulate the game. If I guess a huge large or small number, much larger than I expect the other players to be guessing, that gives me the opportunity to manipulate the average.

If I'm playing only for myself, there's no advantage to doing that. Say there are a hundred players guessing in the range of 0 to 1000, and I come in and guess a googol. Then I've pretty much set how the game will go - the average will be about 1/100th of a googol, far away from all the other players, and it's obvious I can put the average wherever I want - but I don't win because those other players are much closer. I have to pull my own guess far away from the average in order to be sure of controlling the average. I can't rig the game so that I win, but I can rig it so that the player other than me who guesses the greatest (or least) number wins.

So what happens if I have a confederate, I guess a googol, my confederate guesses 1/1000th of a googol, and everyone else guesses in the range 0 to 1000? Then my confederate wins.

It seems like the game is stable as long as people don't collude - we'll all tend to "cooperate" by guessing numbers near where we think the average of cooperating guesses will be, and that will tend to be close to zero because that's the only really distinguished real number.

But if people attempting to collude in pairs is a common enough thing that we expect it to happen at least once per game, then it seems like it will turn into a contest of who can think of the biggest numbers, because a pair wins if they scale their strategy to be bigger than anyone else's.

[identity profile] mskala.livejournal.com 2006-10-06 05:50 pm (UTC)(link)
Quick way to drastically reduce the possibilities for manipulation: make it a contest to guess the median instead of the mean.

[identity profile] mskala.livejournal.com 2006-10-06 06:31 pm (UTC)(link)
Dunno... if you expect guesses to follow a reasonably normal distribution, the mean and median should be the same. I imagine that if you did it with real jelly beans, you'd find that the mean and median of guesses are pretty much equally good estimators of the actual number of beans in the jar; they should be close to identical. The difference is that the mean will include all the wacky far-out guesses and the median won't; so the mean is more susceptible to manipulation. Whether you want that in the real-numbers game depends just depends on what kind of participant behaviour you want.

It'll certainly be interesting to see what kind of actual distribution of guesses you get from this.

[identity profile] wisn.livejournal.com 2006-10-06 06:24 pm (UTC)(link)
My first thought was that you should either only allow positive numbers or announce that all numbers will be entered as absolute values. The jelly bean game assumes the jar, unseen, may have zero jelly beans, but it won't have a negative quantity. The jar, seen, is presumed to have an undetermined but vaguely bracketable quantity (it more than eight and less than a googol; it's assumed not to have a hidden chamber or a pronounced punt in the base), which helps control the guesses.

[identity profile] mskala.livejournal.com 2006-10-06 06:26 pm (UTC)(link)
It would work the same way if my confederate and I used large negative numbers or large positive numbers. As long as the magnitude of the numbers we use is a lot bigger than that of the numbers anyone else is using, it doesn't matter which direction the other pairs are going. The only way that positive and negative manipulations would cancel out in such a way that the biggest manipulators don't win, is if the range of sizes used by the biggest manipulation happens to overlap the range of sizes used by other guessers (manipulative or not).

Near ties would still cause some weirdness even if the biggest manipulations were in the same direction. Suppose my confederate and I choose 1/100th googol and one googol, another pair chooses 1/1000th googol and ten googol, and there are about 98 other guesses all in the range 0..1000. Then the average is about 1/10th googol, and my confederate wins, even though the other pair had the biggest effect on the average.

To make the strategy work, my confederate and I want to guess so that we are sure of having a guess larger in magnitude than anyone else's guess, and a guess larger than that by a factor of more than the number of other guesses, to drive the average into the space between our guesses. We can make our guesses positive or negative and it won't matter as long as they're big enough.

[identity profile] mmcirvin.livejournal.com 2006-10-07 01:23 am (UTC)(link)
Yeah, I think it was a lottery in which you could enter as many entries as you wanted by sending in a card with the number of entries on it, but the prize would be some large sum ($1,000,000?) divided by the total number of entries. So the person sending in the most astronomical number would undoubtedly win, but really the moment anyone sent in a card with a googolplex written on it, nobody would win anything.

[identity profile] mmcirvin.livejournal.com 2006-10-06 10:38 pm (UTC)(link)
If you can pick any number, using the geometric mean might be more interesting. (With perhaps some extra fiddling to deal with negative entries, since otherwise the sign of the result is effectively a crapshoot.)

[identity profile] mmcirvin.livejournal.com 2006-10-06 10:39 pm (UTC)(link)
...And I was going to say that you'd then use the logarithm to determine closeness. But come to think of it, that just turns this contest into the exponential of the existing contest.

[identity profile] plant-geek.livejournal.com 2006-10-07 02:38 am (UTC)(link)
if i win, will you give me a fishbowl full of jellybeans instead of $5?

[identity profile] paracelsvs.livejournal.com 2006-10-07 02:32 pm (UTC)(link)
I'm winning, just so you know.

[identity profile] witsarah.livejournal.com 2006-10-08 11:10 pm (UTC)(link)
Except that I'm going to win.

[identity profile] paracelsvs.livejournal.com 2006-10-08 11:16 pm (UTC)(link)
Yes, well, it's going to be pretty hard to do that when I'm winning.