Returns the value of the gaussian loglikelihood function at params.
Given the whitened design matrix, the loglikelihood is evaluated at the parameter vector params for the dependent variable endog.
Parameters: | params : array-like
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Returns: | loglike : float
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Notes
The loglikelihood function for the normal distribution is
-\frac{n}{2}\log\left(Y-\hat{Y}\right)-\frac{n}{2}\left(1+\log\left(\frac{2\pi}{n}\right)\right)-\frac{1}{2}\log\left(\left|\Sigma\right|\right)
Y and Y-hat are whitened.