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- 13 June 2007
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Hi Howard,
I understand and I suspect that most of us here at ASF also understand the importance of the walk forward methodology using out of sample data in verifying the robustness of the system going forward. The concept is a very sound one and we thank you for your patience.
The one area that I personally like to see being discussed more is this concept of system decay, and how over time, the system will lose its edge and will ultimately fail. Logic would indicate (to me at least) that as more and more people trade a particular type of system, say a 30 day breakout system, that the system would have a greater chance of succeeding as an ever increasing number of traders initiate the trade upon confirmation of the breakout, to a point where it almost becomes a self-fulfilling prophecy. How then does such a system lose its edge over time ?
Does a system tend to fail as more traders use it ?? If the answer is yes then can one conclude that the system would more effective if it is used by fewer traders ?
When a system starts to lose its edge (presumably from over-exposure from too many traders using it) the number of traders using it will also tend to decrease over time as they seek out other more profitable strategies. When the number of traders have dropped off sufficiently, will the edge that the system had initially return ?? If the answer is NO, then what residual external factor that had caused the system to initially fail has remained in play to continually impact on the system even though the number of traders using it has dropped off dramatically ?
I can’t get my head around this system decay concept and hope that you may be able to shed some light on the subject.
Hi Bing --
The post I just made to points Sir Burr raised address some of your question.
To illustrate why having more traders trade a system makes it more difficult for any one of them to make a profit, think about what the order book looks like. Say a stock is trading at $100 and headed for $105. The best bid may be for 200 shares at 99.98 and the best offer for 100 shares at 100.02. There are other offers of 100 shares each at 100.05, 100.10, 100.25, and 100.50. There are more bids, but they are lower and will not play here.
Say there is one buy signal from a program that is generally correct. 100 shares are bought at the market -- 100.02.
Instead, say five buys signals arrive. Those trades take place at 100.02, 100.05, 100.10, 100.25, and 100.50.
Eventually, the system issues a sell signal. The same thing happens, but now with prices on the bid side. One order gets filled at the best bid, the other get worse fills.
Wait, I hear you say, I use limit orders to avoid that slippage -- I'll buy at 100.06 limit. If your order came in third, your limit order of 100.05 never got filled and you get no profit at all.
My point is this -- the more systems that want to buy this stock, the worse their average performance will be.
Thanks,
Howard