Description:
        Calculates the Mean Squared Error (MSE) of
        an estimation on flat numpy ndarrays.
    Parameters:
        mean:      actual value (numpy ndarray)
        estimator: estimated value of the mean (numpy ndarray)

def mse(mean, estimator):
    """
    Description:
        Calculates the Mean Squared Error (MSE) of
        an estimation on flat numpy ndarrays.
    Parameters:
        mean:      actual value (numpy ndarray)
        estimator: estimated value of the mean (numpy ndarray)
    """
    return np.mean((np.asarray(estimator) - np.asarray(mean)) ** 2, axis=0)