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ouyangwulongfollowshare
5-17-2007 11:59 AM
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Here's a layman's explanation of how certain examples can seem to defy probability:

Suppose you flip one coin 10 times. You should probably get around five heads and five tails. If you flip 10 coins at the same time, you should get about the same. However, the results only consistently approach that ratio as the volume of flips increase.

What if you flipped 10 coins twice? You should get around 10 heads and 10 tails. However, if by coincidence, you flip 10 heads on the first flip, your next flip becomes more likely to flip more tails in order to preserve probability.

If you do this with a lot of coins (say, 100) over a long time (say 1,000 flips) this trend becomes exagerated. Some coins begin to flip more consistently tails, and thus other coins must flip more consistently heads to balance out the system.

This could be creatively interpreted to explain why some people are "unlucky."
9 Comments   | Add a Comment
5-17-2007 6:47 PM
kmcolo
So you are saying that, after flipping 10 coins all heads, the probability of flipping more tails than heads on the second round is greater than if you had not flipped all heads?
5-17-2007 7:16 PM
jklugman
Ouyang Wulong made a mistake when he said that if you flip a coin ten times and come up with all heads, for the next set the coin is more likely to come up tails to "balance out" the first set.

The probability of flipping a fair coin ten times and coming up with all heads is 1 out of 1024. It doesn't matter if this is your first set of 10 coin flips, your second, or your third.

I think the point he was making is that in the short run, you can get weird results (like flipping a coin 10 times and getting heads all ten times) but in the long run random phenomena take on a predictable pattern (if you flipped a coin a million times, 50% the time you are going to get heads, 50% the time you a...
5-17-2007 10:01 PM
ouyangwulong
In response to your concerns about the independence of probability, you aren't considering that the entire system also needs to conform to the 50% probability as well. The farther a system strays from it's natural probability the less likely it is to remain that way. More important that your individual coin flipping approaching 50%, is that all coin flipping ever also approaches 50%.

The odd results are actually, more often that individual coins will flip more heads than they should, while other individual coins flip more tails than the should, although the system maintains its 50% equilibrium. There are some cool java applets out there that illustrate this.

The reason for the gambler's fa...
5-17-2007 10:03 PM
ouyangwulong
To clarify, yes, you are exactly right. We're talking about weird results in the short term, and why they don't violate probablity on the whole.

My first example was perhaps not as clear as it could have been.
5-17-2007 10:44 PM
ouyangwulong
Actually, an even better way to put it, tying it in to arifsali's clip, good or bad luck could be viewed as a product of continuity in our individual perspective. A coin's behavior in a group is observable to us (although it isn't easy to keep track of the individual coins if you try this...) but only as part of the whole. The coin does not have its own perspective, and does not evaluate its own flips.

Thus, the system can be analyzed in terms of unique tests, or events. However, when it comes to people we are inordinately cognizant of our own happenings and fail to consider them as part of a larger system. Thus, sometimes strange results for any one individual can seem like good or bad luc...
5-17-2007 10:46 PM
ouyangwulong
Sorry, I meant to hypertext arifsali's clip, but it slipped my mind...
5-17-2007 10:48 PM
jklugman
In response to your concerns about the independence of probability, you aren't considering that the entire system also needs to conform to the 50% probability as well. The farther a system strays from it's natural probability the less likely it is to remain that way.
Look at the formula in your clip--it doesn't say anything about balancing out previous results, or correcting a system that has "strayed" from its natural probability.

The probability of flipping a coin ten times and getting 10 heads is .5^10 ("p^k"; the other terms simplify to 1 because both n and k are 10). If you do a set of 10 coin flips and get 10 heads, your chances of getting 10 heads on the next set ar...
5-17-2007 10:52 PM
ouyangwulong
We agree. I clipped this in response to a posting discussing why some people seemed to be lucky or unlucky. If you get a chance, you can conduct this experiment in person, or (much more easily) in Java, and you may find the results very interesting.
5-18-2007 4:18 AM
ouyangwulong
Thanks to you guy's constructive scrutiny, I've decided to reword my argument. I haven't changed my mind, but I think I can give more precise details to explain what I'm thinking...

However, first I'll need to get some sleep and some coffee...

Keep watching, though...
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