My example was per drawing [per day]. Let's just say your odds are 1 in 1000 of being selected on any 1 day. Over a 20 day period, your odds become additive --

1/1000 + 1/1000 .... or 20/1000 or 1 in 50.


>Mark,
>
>I almost understood the lottery situation :).
>Now consider this situation:
>The total number is 200
>You should select 2
>The time period is 20 days
>
>The probability for each day is ??????
>2/200 *1/199 - according your formula
>1/200+1/199 - according mine (now I seriously doubt in this formula)
>
>Now, for the whole time period probability is ?????
>?
>?
>
>
>
>>Nope. You have 2 chances that the selected "ball" is your "number." To get it, look at the lottery analogy. I have picked 6 numbers. I have 6 chances that the first ball drawn, out of a 50 possible balls, will match 1 of my 6 numbers. I now have a match on the first draw. That was 6 out of 50. Now 49 balls and 5 numbers remain. I next have 5 chances that the next ball will match one of my 5 remaining numbers or 5 out of 49. So the progression I gave you is correct for the lottery. Transferring that logic to the beauty contest is valid.
>>
>>>Mark,
>>>
>>>>If 2 are selected then the probability is:
>>>>
>>>>2/n * 1/[n-1]
>>>
>>>This is obviously not right. If the possibility of one to be selected of n is
>>>1/n (obvious), then the possibility to be one of 2, who selected, should increase (not decrease, as in your formula).
>>>so, it should be 1/n + 1/[n-1], as I understand.
>>>
>>>
>>>
>>>>In our State Lottery, you have to pick 6 out of 6 numbers from a total of 50 numbers, that probability is:
>>>>
>>>>6/50 * 5/49 *4/48 * 3/47 * 2/46 * 1/45 or about 16,000,000 to 1 for a payout of only $4,000,000 or less.
>>>
>>>Ok, it's probably right.
>>>
>>>What about other kind of lottery, there each number could be from 0 to 9,
>>>say 000000
>>> 999999
>>>any number between them.
>>>The probability is 10 in -6, as I understand, so greater, than in State Lottery, which also seems to be non-obvious. Or am I wrong?
>>>
>>>>
>>>>I am sure someone can provide the simple formula, like x = n!/[[y-1]**[n-1]] -- which this example is not correct. It's been about 15 years since I had Differential Equations.
>>>>
>>>>>So, I may hope? :) BTW, have you already been the Member of the day? I don't remember...
>>>>>
>>>>>So, the probability may be calcualted as: (right?) in each particular day for each such person:
>>>>>prob=1/337+1/336, if all these people had access in the last 20 days in each day.
>>>>>
>>>>>Let's count for simplicity 2/200 or 1/100.
>>>>>
>>>>>This feature started from Feb. 19th, say, 20 days.
>>>>>How will be prob calculated in this case? I never studied theory of probability seriously :(
>>>>>
>>>>>TIA