Mike Raiford wrote:
> Hey, whatever happened to those, Andrew?)

Now there's a pretty random wondering!

The answer, of course, is that usually when I think of something, I'm 
nowhere near a computer.

For example, just the other day I was on the London Underground, and I 
wondered: How the **** do they get the trains onto the track? Something 
like the Circle line is completely subterranian, and I'm pretty sure the 
factory where they make the carrages isn't. And it's not like you can 
just take a train carrage down the escalator with you...

> So, here it is. Stupid Probability Question #1:
> 
> Suppose you're on a 100 year flood plane. Now ... suppose the last flood 
> was some 76 years ago in that area. Does the probability of a flood 
> increase as your approach 100 years, or does the probability of a flood 
> stay the same?
> 
> -OR- is probability irrelevant due to changing conditions?

In the first analysis, the probability of an event remains constant 
regardless of past events. E.g., the probability of rolling a 6 with a 
dice is *always* 1/6, regardless of how long it is since the last time 
somebody rolled 6 (or any other number).

This analysis works only if events are statistically *independent*, of 
course. In the case of a flood, they might not be.

In the second analysis, if a dice is rolled and almost never turns up a 
6, this could be taken as evidence that the dice is biased, in which 
case the probability is not actually 1/6.

In the case of a dice, we can analytically compute what the probability 
"should be". In the case of a flood, presumably you can't. You can only 
*estimate* that probability. And it is likely that different estimation 
methods will yield different estimates. In short, the "probability" 
becomes a rather subjective thing.

On top of all that, of course, probability tells us nothing about when a 
particular area will flood. It tells us only how many times it will 
flood over a long period of time. Probability only talks about large 
numbers, not specific instances.

It is *highly* unlikely that any specific person will win the lottery. 
And yet people still win, somehow.

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