I have developped a "profitable" system upon ym's last 3 months of data (from 9th Mars to First Juin). The percent profitable is 40.87% and the profit factor is 2.1. While the performance report looks all beautiful, I know that this isn't reliable yet since the amount of data is quite limited. My questions to those experienced system traders are the following: What is the minimum amout of data need to backtest? Since the market is changing, would the old data (like those older than a year) biase the developpement of a system ? In another word, Should I just use the most recent data to develop my system in order to approach the most recent tendency of the market ? If so, I will have to update the parameters of my system often, like every week or every month, right? Thank you!