Suppose X measures the interval until the rth success and with parameter \(\lambda\) as the average interval until the 1st success. Then X with probability function
\begin{equation*}
f(x) = \frac{x^{r-1} \cdot e^{-\frac{x}{\lambda}}}{\Gamma(r) \cdot \lambda^r}
\end{equation*}
has a Weibul Distribution. (Note, this is a special case of the Gamma Distribution and is sometimes called the Gamma Distribution.)