Exponential distribution.
Its probability density function is
for x > 0 and 0 elsewhere. is the scale parameter, which is the inverse of the rate parameter . The rate parameter is an alternative, widely used parameterization of the exponential distribution [R168].
The exponential distribution is a continuous analogue of the geometric distribution. It describes many common situations, such as the size of raindrops measured over many rainstorms [R166], or the time between page requests to Wikipedia [R167].
Parameters :  scale : float
size : tuple of ints


References
[R166]  (1, 2) Peyton Z. Peebles Jr., “Probability, Random Variables and Random Signal Principles”, 4th ed, 2001, p. 57. 
[R167]  (1, 2) “Poisson Process”, Wikipedia, http://en.wikipedia.org/wiki/Poisson_process 
[R168]  (1, 2) “Exponential Distribution, Wikipedia, http://en.wikipedia.org/wiki/Exponential_distribution 