I took a long look at data for about 1000 stocks over a 6 month period, and found a mathematical formula that I believe works well to make money. It does not rely on being able to buy and sell quickly (a 5 or 10 minute lag would not break the system).
I took that system and applied it to all NYSE stocks listed on Yahoo finance since 1962. It works very well in most years (does poorly in 2004, 2008, 2015), and the good years more than compensate for the bad years.
I am aware Yahoo Finance has a survivorship bias. Although I don't have hard numbers to back this up, I don't believe my system would be drastically affected by this. My system is not picking out companies that are more prone to failure than the average publicly traded company. I manually simulated what would happen if a few companies happened to go completely bankrupt immediately after I bought them, and it doesn't affect the numbers drastically due to diversification.
I've spent countless hours looking through my work for mistakes. My girlfriend (a math major) has also looked through it to try to find errors. I've also talked through the basic principles of the system with a few people I trust, and they can't pick out any flaws with it.
As a trial run, I invested $100 using the system. So far (3 weeks in), it has been behaving as I would expect it to behave.
What can I do to further stress test my system? The returns are significantly larger than mutual funds or indexes, so I'd like to put the majority of my expendable money into this. And at the same time, I want to do my due diligence in making sure that I'm not overlooking anything obvious.