Before revision:  We aim at understanding the impacts of order flows on the weak-form efficiency through computational experiments based on an empirical order-driven model  .
After revision:  We aim at understanding the impacts of order flows on the weak-form efficiency through computational experiments based on an empirical order-driven model called the modified Mike-Farmer (MMF) to predict the impacts of order flows on the autocorrelations in ultra-high-frequency returns, quantified by Hurst index H_r .