'Probit (Normal) probability density function
Parameters
X : array-like
The linear predictor of the model (XB).
Returns
pdf : ndarray
The value of the normal density function for each point of X.
Notes
This function is just an alias for scipy.stats.norm.pdf'

def pdf(self, X):

    X = np.asarray(X)
    return stats.norm._pdf(X)
