Probability Question

Biochemist

United States1008 Posts

May 07 2009 01:28 GMT

So I'm trying to help someone with their statistics class, but I've never taken statistics so I'm trying to teach myself from their crappy crappy textbook. We're doing binomial distributions with the expression

P(x=r) = nCr * P^r * (1-P)^(n-r)

Now I can just plug shit in and get the right answer, but that's not the point since I'm trying to understand what all these terms mean. I understand the nCr and the P^r, the former being the number of ways to get r out of n objects (i.e. there are 15 ways to get 2 different patients out of a pool of 6 patients). P^r is of course the product of the probabilities (i.e. if each patient has a 60% chance of surviving, 0.6^2 gives the probability of two patients surviving). What I don't understand is the (1-P)^(n-r) part. What is it doing, and why is it needed?

Thanks!

goldrush

Canada709 Posts

May 07 2009 01:31 GMT

If r out of n objects with probability P of doing some action happen to do the action, then n-r objects must NOT do the action. Which has probability of 1-P. And so, by the product rule (assuming independence, etc.), you get (1-P)^(n-r)

Biochemist

United States1008 Posts

May 07 2009 01:37 GMT

Ah, of course. It was staring me in the face.

R3condite

Korea (South)1541 Posts

May 07 2009 04:46 GMT

On May 07 2009 10:28 Biochemist wrote:
So I'm trying to help someone with their statistics class, but I've never taken statistics so I'm trying to teach myself from their crappy crappy textbook. We're doing binomial distributions with the expression

P(x=r) = nCr * P^r * (1-P)^(n-r)

Now I can just plug shit in and get the right answer, but that's not the point since I'm trying to understand what all these terms mean. I understand the nCr and the P^r, the former being the number of ways to get r out of n objects (i.e. there are 15 ways to get 2 different patients out of a pool of 6 patients). P^r is of course the product of the probabilities (i.e. if each patient has a 60% chance of surviving, 0.6^2 gives the probability of two patients surviving). What I don't understand is the (1-P)^(n-r) part. What is it doing, and why is it needed?

Thanks!

u shoudl be able to get it since P^r is of course the product of the probabilities

1-P)^(n-r) would be... product of the probabilities of that the rest don't happen

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