How to Train a Predictive Model

I wonder how much we're gonna see of people building predictive based things, you know, for stocks or whatever it happens to be and how accurate those models will be. I don't know. It's really hard to know. Part of the training process is you peel off some of your data, usually about 20%. And once you've trained the model, you test it against the data that it's never seen before. So, if it gets them right and you know what the answer is because it's real data, then you're like, okay, this is good. If it gets them wrong, then you go back and you refine your initial training. You need to make sure when