Poisson model Hessian matrix of the loglikelihood
Parameters : | params : array-like
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Returns : | hess : ndarray, (k_vars, k_vars)
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Notes
\frac{\partial^{2}\ln L}{\partial\beta\partial\beta^{\prime}}=-\sum_{i=1}^{n}\lambda_{i}x_{i}x_{i}^{\prime}
where the loglinear model is assumed
\ln\lambda_{i}=x_{i}\beta