Poisson data model:
\(Y =\) number of independent event that occur in a fixed amount of time
\[Y|y \sim Pois(\tau) \]
Probability mass function:
\[ f(y|Y) = \frac{\tau^ye^-\tau}{y!} \quad for y \in \{0, 1, 2, ...\} \]
(sum to 1)
\[E(Y|\tau) = Var(Y|\tau) = \tau \]