# If something happens to 1 in 100 persons, is the chance of that person being you 50/50?

There isn't really much to add but since the site insists at least 30 chars body, I am asking again : If something happens to 1 in 100 persons, is the chance of that person being you 50/50?

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Clearly not. If you’re one of a group of $100$ people, and one of you is to be chosen at random to be shot at dawn, your probability of surviving past dawn is $0.99$, not $0.5$. –  Brian M. Scott Dec 26 '12 at 13:38
There are 200 people in one of my classes. (horribly many). The teacher (uniformly) picks random students and asks him to answer a question. After about 60 students, I was picked. So for most people, it'snot 50/50. If your teacher has $1/2$ chance of picking you and $1/2$ chance picking from the rest, then it's 50/50. So, it depends on your distribution and how you ask questions. –  FrenzY DT. Dec 26 '12 at 13:39
ok, so if I have no knowledge of the distribution, is it wrong to assume a 50/50 chance? What will it be then? –  phil Dec 26 '12 at 13:45
@phil Nbody an tell you what you are supposed to believe. –  Michael Greinecker Dec 26 '12 at 13:54

$$\frac{1}{100}=0.01=1\%.$$
An example for the 50/50 chance would be a model with balls and bins. Assume we have one bin and $n$ blue and $n$ red balls in there. You are allowed to take one ball without explicitly looking which color it has - the probability of it being blue/red will be both $50\%$ because $n$ out of $2n$ balls have the same property (here the color). If you want to repeat this experiment keep in mind to put the ball back, otherwise you would get other probabilities!
@phil: So let's say you have $n$ blue and $k$ red balls without knowing what $n$ and $k$ are... yeah, we can't do anything because we only know that we have $n+k$ balls, but we need at least one of the numbers to be able to provide information about the problem and how to handle it. –  Christian Ivicevic Dec 26 '12 at 13:55