Poisson model probability mass function
Parameters : | X : array-like
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Returns : | pdf : ndarray
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Notes
The PMF is defined as
\frac{e^{-\lambda_{i}}\lambda_{i}^{y_{i}}}{y_{i}!}
where \lambda assumes the loglinear model. I.e.,
\ln\lambda_{i}=x_{i}\beta
The parameter X is x_{i}\beta in the above formula.