# Number of tries needed for occurrence of a certain event

Discussion in 'Probability' started by Shahin, Jul 5, 2010.

1. ### ShahinGuest

Hello all,

I was wondering is there any way to determine the number (or
minimum number) of tries needed for occurrence of a certain event? for
example, how many times we need to flip a fair coin to get a head?
what information we may need for this?

Thank you,
Shahin

Shahin, Jul 5, 2010

2. ### Frederick WilliamsGuest

Theoretically there is no upper limit on the number of times you may
need to flip a fair coin to get a head.

Frederick Williams, Jul 5, 2010

3. ### Ray KoopmanGuest

Look up the "negative binomial distribution".

Ray Koopman, Jul 5, 2010
4. ### Stephen J. HerschkornGuest

Or, more simply, the geometric distribution.

Stephen J. Herschkorn, Jul 5, 2010

I'm reminded of a question I was asked for a job once. There are 3
different colored marbles in a bag (unknown, but large amount). How
many times must you draw until you're guaranteed to have 2 of the
same color?

Applying this to your question, it really depends on what kind of
probability you're talking about. Some situations (such as a flip of a
coin) have no upper limit. Others (such as the one I just presented)
do.

link, Aug 9, 2010