Exponential Distribution

$$f(x)=\{\begin{array}{ll}\lambda e^{-\lambda x},& x\ge 0\\ 0,& x<0\end{array}$$

Expected Value

$$\mathbb{E}[X]=\frac{1}{\lambda }$$

Variance

$$\mathrm{Var}(X)=\frac{1}{{\lambda }^2}$$

See also: Exponential Likelihood