In this paper, different monetary policy rules are re-estimated employing real-time data, known as Greenbook data, through MLE and Bayesian Estimation. The results favour the rule with persistent shocks and show that the inertial movement of the Federal funds rate is an illusion, as mentioned by Rudebusch (2002). Since it would be possible to have an omitted variable bias due to impure serial correlation, however, the rule with persistent shock could imply that the other serially correlated variable is required to explain the inertial movement of Federal funds rates. Based on this idea, serial correlation tests are implemented, showing the possibility of an omitted serial correlated variable. Therefore, it is reasonable to search for other persistent variables. As an illusion implies unpredictability in the movement of the Federal funds rate, this hypothesis could be against the credibility of the central bank. In order to have a reasonable explanation for the inertial movement other than persistent shocks, therefore, modifying the rule with a 3-month t-bill rate is re-estimated. The result shows that the rule with the t-bill rate outperformed in all aspects and is compatible with one theoretical explanation of minimizing the financial volatility for inertial movement.