use F test to test whether restricted model is correct
restricted : Result instance
f_value : float
p_value : float
df_diff : int
See mailing list discussion October 17,
This test compares the residual sum of squares of the two models. This is not a valid test, if there is unspecified heteroscedasticity or correlation. This method will issue a warning if this is detected but still return the results under the assumption of homoscedasticity and no autocorrelation (sphericity).