Abstract
We know that investors can observe some information containing in the order flow. A trade is dealt with both sides of buyers and sellers. Some traders have inside information while others may not. The asymmetry of information will apparently inference the investment decision and then affect the stock volume, price volatility and other market parameters such as the market liquidity. We build a microstructure model of trading to discuss the relation between the order flow and the arrival rates of informed and uninformed trades. Then, using the estimated parameters, we calculate the probability of information-based trading(PIN) which can be used to forecast the bid-ask spread. We find that the PIN in the futures market is significantly bigger than that in the stock market. In addition, PIN can not only explain the bid-ask spread in the stock market, but also in the futures market in Taiwan.