# Finding the number of trials needed to obtain an outcome that is not equal to zero

• Nov 22nd 2009, 05:18 PM
libragirl79
Finding the number of trials needed to obtain an outcome that is not equal to zero
Consider independent trials each of which results in outcome i, i=1,2,...k with probability p of i. Let N denote the number of trials needed to obtain an outcome that is not equal zero and let X be that outcome. Find P(N=n) where n>=1
Thanks so much!
• Nov 22nd 2009, 05:32 PM
matheagle
This sounds like a geometric with probability of success $p=p_1+p_2+...+p_k$

$P(N=n)=p(1-p)^{n-1}$

I think I had my p's and q's backwards, I misread the NOT equal to zero.
And what's X doing here?