Lizgdal: I need at least a 10-year backtest for my WWL system, and 20 years for new screens. But, maybe AI will change that.
I used a WWL system for several years, my logic was what’s working lately that has also worked long term which needs both recent and long term to make the decision. I now have a whole series of AI simulations using 20 years of data where I train multiple models for 4 to 17 years, after a gap to prevent data leakage verify performance for the following year. Outperformance in general but not guaranteed begins after 5 or 6 year of training.

Your link to Yuval Taylor’s P123 blog above is a good example of how much effort he puts into his studies.
His end of year 2025 post on his performance.
“””I had an excellent year.
My hedge fund, Fieldsong Investments, was up 59.54% for the year. My private foundation was up 24.94%. Both of these use leverage and are hedged.
My wife's and my retirement accounts that are not invested in the hedge fund were up 34.59%. My kids' accounts were up 59.60%. These accounts don't use leverage and were not hedged.
Altogether I made $3.93M. And all of this was using Portfolio123 as the basis for all of my decision-making. Thank you P123!
I also manage two other accounts using Portfolio123 and IB. One was up 60.91% and the other was up 28.21%.
“””
How did your 2025 P123 portfolio perform? - General - Portfolio123 Community

I’ve been using P123 for years and believe it is the best site available for factor investing however only a small percentage of the investors there do as well as Yuval. But Yuval is obviously a very intelligent individual and has been doing this full time for several years. The more successful individuals tend to have either a strong background in statistics, finance or software.