0.24 R uses the convention that k is the number of failures, so that the number of trials up to and including the first success is k + 1. The following R code creates a graph of the geometric distribution from Y = 0 to 10, with p = 0.6. Y=0:10 plot(Y, dgeom(Y,0.6), type="h", ylim=c(0,1), main="Geometric distribution for p=0.6", ylab="Pr(Y=Y)", xlab="Y=Number of failures before first success") Geometric distribution using Excel The geometric distribution, for the number of failures before the first success, is a special case of the negative binomial distribution, for the number of failures before s successes. The Excel function NEGBINOMDIST(number_f, number_s, probability_s) calculates the probability of k = number_f failures before s = number_s successes where p = probability_s is the probability of success on each trial. For the geometric distribution, let number_s = 1 success. For example, =NEGBINOMDIST(0, 1, 0.6) = 0.6 =NEGBINOMDIST(1, 1, 0.6) = 0.24 Like R, Excel uses the convention that k is the number of failures, so that the number of trials up to and including the first success is k + 1. See also Hypergeometric distribution Coupon collector's problem Compound Poisson distribution Negative binomial distribution References External links Geometric distribution on MathWorld. Discrete distributions Exponential family distributions Infinitely divisible probability distributions Articles with example R code