Blurb::
The parameter for the Poisson distribution, the expected number of events in the time interval of interest
Description::
The density function for the poisson distribution is given by: 
\f[f(x) = \frac{\lambda e^{-\lambda}}{x!}\f] 
where \f$\lambda\f$ is the frequency of events happening, and \c x is the number of events that occur. 
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