And lo On Tue, 29 Jun 2010 13:13:47 +0100, Mike Raiford  
<"m[raiford]!at"@gmail.com> did spake thusly:

> (Not that I plan any more.. but since this is my first stupid  
> probability question. This is sort of in the vein of Andy's classic  
> Random Wonderings post series ...Hey, whatever happened to those,  
> Andrew?)
>
> 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?

No

> -OR- is probability irrelevant due to changing conditions?

Kind of :-)

As well as the other answers given think of which hundred year span you're  
thinking of. If I date it from the last flood then in one hundred years I  
have one other flood then in the next hundred years another I have 1 flood  
per century. If I start at the same point and have two floods in the first  
hundred years and no floods in the second hundred I still have 1 flood per  
century.

What in this instance the term means is that you can expect to have just 1  
flood per century, if you get more, bad luck and some reassessing of your  
insurance might be done. if not good luck; keep paying us the same amount  
thank you very much.

It's just as a species we like patterns, we like cycles and we don't like  
randomness, so as roughly mentioned if I flip a coin 8 times and it comes  
down heads each time the probability of it coming down tails doesn't  
increase, but it might mean it decreases as the coin is biased hence the  
"kind of" on changing conditions. If it's not biased then over time the  
ratio comes out to 1:1, but if I just pick a section of my results it  
might not seem that way.

-- 
Phil Cook

--
I once tried to be apathetic, but I just couldn't be bothered
http://flipc.blogspot.com

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