Gamma becomes normal as \(r \rightarrow \infty\text{.}\) Assume that the average time till a first success is 12 minutes and that \(r = 8\text{.}\) Then, the mean for the Gamma distribution is \(\mu = 12 \cdot 8 = 96\) and \(\sigma^2 = 8 \cdot 12^2 = 1152\) and so \(\sigma \approx 33.9411\text{.}\)