Quote from braincell:
OK well, I agree for the most part. I am probably assuming differently though, because I am working on daytrading ATS which executes around 10 trades per day on a single underlying. In my experience (coupled with a bunch of studies, tests, etc), the small-scale signals and rules that traders use on such a short time-frame have not changed for a very long time. True, there are some effects like 1) and 2) you mentioned that make the markets "open" in that way, but looking at intra-day data, not much changes because of them. Especially if you are focusing on a very general de-coupled and unspecific underlying such as SPY ETF, the ES futures, or even the USDEUR. On smaller time-frames like this, you also solve the first problem you mentioned: lack of data. With a 10-year backtest, with 10 trades per day average, that amounts to around 26000 trades to look at, and this is in my opinion enough data. The longer the time-frame (of having open positions), the less significant the results, so I probably made a mistake by not mentioning my assumption regarding the ATS frequency.
Still, you are slightly wrong. A good system will incorporate all the data which is affecting the markets. Such systems are hard to come by, but they exist. That includes incorporating exchanges changing rules, margin requirements, or whatever consequences there may be due to any of those. Systems exist that model the changes such rules will probably have on other systems that trade a specific instrument or derivative. They are probably not something an average person will code within a few months, but it's doable. Often, they require human input though (such as how something psychologically affects the market participants) so it's a half-baked argument I'm presenting.
Anyway, my main point was that if you are developing a system for daytrading which does something like 10 trades per day at least, the rules that exist on those intra-day 1minute charts hardly change. This is the first reason why an ATS is best developed for this timeframe. The other is that it gives you a huge amount of data to work with, thus increasing the probability of your hypothesis testing (backtesting) projecting forwards. This much I suppose we can agree on, and it is my advice that everyone should develop an ATS within such smaller timeframes.