The confusion here is based on what question is being asked. The probability of flipping a coin heads up is 50%, and the probability of doing it twice in a row is 25%. If you flip a coin heads up on your first toss, your probability of flipping it heads up on your second toss is 50%. Wait what? I thought I just said that flipping heads up twice in a row only had 25% probability. Yes, but in the suggested scenario, I already flipped the coin for the first toss, and know the outcome, thus I eliminate all the possible scenarios in which my first toss was tails up, with half the possibilities eliminated I’m back at 50%.
WapitiBob’s calculations are correct. He is examining the probability that something will occur at least once over a series of n trials, which is equal to the percentage of times the event would occur if he examined n number of events an infinite number of times.
Aperventure is changing the question and examining the probability that something will occur one time, in a single trial, which is equal to the percentage of times the desired event would occur over an infinite number of events.
So more simply, Bob’s calculations answer the question, “if I pick ten marbles from a jar, how often is at least one of them blue?” And Aperventure’s answers the question “if I choose one marble from a jar, how often is it blue?” They are two different questions.
Obviously the percentage of time that the desired event occurs at least once when you repeatedly select a bundle of events(WapitiBob’s examination that something will occur at least once over a period of trials) is much higher than the percentage of times that the event occurs.
A scenario that exemplifies the importance of asking the right question is the old game show where the contestant is asked to choose one of three doors, then the host reveals a “loosing” door and the contestant is asked if he wants to change his original choice. One must be careful with what question they answer. What is the contestant’s odds of choosing the correct door on the first choice? 33%. What is the contestant’s odds of choosing the incorrect door on the first choice? 66%. If the contestant chooses to toss a coin to decide whether to leave his original choice changed or unchanged what are the odds of winning? 50%. Why? The contestant who chooses to stick with the original choice MUST choose CORRECTLY on the first choice to win. The odds of choosing correctly are 33%. The contestant that chooses to change his door on the second choice MUST choose INCORRECTLY on the first choice.(the host will reveal a non-winning door, so in order to move to a winning door, the contestant must have chosen incorrectly initially) Thus his odds are 66%. What is actually happening is that the contestant is choosing whether to win by picking the winning door(stay), or to win by picking the loosing door(change). By using a coin instead of logic to decide how to “make the second choice” half the second choices will be stay(33%) and half the second choices will be change(66%)
(.66*.5)+(.33*.5)=.5 and thus the odds would be 50%, which is also the odds choosing the correct door in the second round.