Investing is basically not that difficult. You pick stocks you like and buy more if they get cheaper. Hang on for the ride and use common sense. I have written several articles about what not to do. But here is one trick that can help you pick winning stocks: use probability theory.What? I am not a math whiz, you say. I can’t add two numbers together, much less use probability analysis, your mind says to me now. It’s hard enough to figure out what stocks to invest in. Don’t give me more work.I understand. But let’s get real here. You use probability theory every day. You estimate whether it’s going to rain before you take an umbrella out. Input: news from the TV on the weather, or turn to weather.com (as long as you are not in Texas these days). Output: estimate the probability you will need an umbrella. Actually, just to be safe, you take one anyway. In other words, if the probability was greater than 50% in your mind, you take the umbrella.The same thing occurs when you invest in a stock. You may like a name like Tesla or Facebook. But now you have to decide how much to buy. That involves your own private probability of how fast and how far they will rise.Of course, you implicitly allow for a drop. Adding these estimates in your mind, your gut tells you how much to invest. That is using probability.Here is how I make the probability process a little more cut and dry in my investing process.The trick is to watch a stock long enough to believe that the probabilities it will rise are greater than the probabilities it will fall further.There is no right answer. There is no grade. There is no site to check your…